Aeration and dissolved oxygen

Having a sufficient level of dissolved oxygen is the most fundamental requirement of all aerobic aquatic organisms, and maintaining a high dissolved oxygen level is an essential of fish-keeping. It might appear that all you need to maintain oxygenation is an air pump and diffuser, but the dissolved oxygen content of water is dependent upon the interaction of many variables, and is not always easy to either predict or measure. Because of the difficulty of obtaining a direct measurement of oxygenation, I’m going to try to list all the relevant factors that contribute to aquarium aeration. 

Wikipedia,
I’ve tried to link into external sources for all the sections, and for this I’ve mainly used Wikipedia.This is not because it is necessarily the best reference, but because Wikipedia has stable URL’S, meaning that links should carry on working for as long as this document remains on the web. In my opinion, the scientific information in Wikipedia is mainly accurate, but it must be remembered that Wikipedia is an open source encyclopaedia, and can be edited by any-one.
wikipedia

Contents


10 points to remember:

  1. Warm water holds less oxygen than cooler water.
  2. Deep tanks have less surface area per volume, and are more likely to become de-oxygenated.
  3. Fish may be suffering from oxygen deprivation long before they show respiratory distress.
  4. Signs of respiratory distress may be related to the carbon dioxide (CO2) or nitrite (NO2) content of the water, as well as the amount of dissolved oxygen (O2).
  5. All the organisms in the aquarium respire, including plants.
  6. All the organisms in the aquarium potentially contribute to the bio-load, not just the fish.
  7. The decomposition of organic material requires oxygen, often a greater amount of oxygen than respiration does.
  8. Nitrification, the biological oxidation of ammonia (NH3) to nitrite (NO2-) and then nitrate (NO3-) is an oxygen intensive process.
  9. Rheophilic fish, those from rapidly flowing, highly oxygenated water, have a higher oxygen requirement than fish from still waters.
  10. Air bubbles don’t contribute much oxygen to the water directly by diffusion.

Oxygen solubility

The atmosphere or “air” contains approximately 21% oxygen (O2), but the solubility of oxygen is influenced by several factors, air pressure, salt content (water with a high TDS can hold less oxygen) and, probably the one that is most relevant to fresh water aquarists, water temperature.
How much oxygen is present will depend upon the temperature of the water, the cooler it is, the more oxygen it can hold. Pure water (H2O) should be fully saturated with oxygen (O2), and in equilibrium with atmospheric oxygen.

Table 1:pdf format
Oxygen solubility (milligrams O2 per litre) at mean sea level & standard atmospheric pressure (STP) which is 760mm or 29.92” of mercury (Hg) or 1013millibars (mb)).

Temperature °C

 O 2 mg/l (100% saturation)

24.0

8.3

25.0

8.2

26.0

8.1

27.0

7.9

28.0

7.8

29.0

7.7

30.0

7.5

31.0

7.4

32.0

7.3

33.0

7.2

34.0

7.0

The effect of both atmospheric pressure and conductivity are smaller, but certainly for pressure relevant. At 4,590m (18,000 ft), atmospheric pressure is only 50% of that at mean sea level. For example in Denver, Colorado (at 5,281 ft or 1,609 m), a measured barometric pressure of 760mm Hg would equate to only 626mm Hg at sea level (The atmospheric pressure at this altitude is only 0.83 (83%) of 1 atmosphere)) and at 29oC this gives a 100% oxygen saturation of only 6.34 milligrams O2 per litre.

Table 2:
Oxygen solubility (milligrams O2 / litre), conductivity and air pressure.

Temperature 29.0°C

O2 mg/l

Conductivity 0mS

7.7

Conductivity 2000mS.(1280 TDS)

7.65(correction factor for 2000 micro(m)S = 0.994 or 99.4%).

Air pressure 700 mmHg (933mb)

7.0

Air pressure 760 mmHg (1013mb)

7.7

Air pressure 800 mmHg (1053mb)

8.0


Processes leading to oxygenation.
There are 2 major processes leading to oxygenation:
  • Diffusion from the atmosphere,
  • and as a by product of photosynthesis by aquatic plants, including algae and cyanobacteria.

Diffusion
The diffusion of oxygen from the air occurs naturally at the surface of the water. The oxygen content of the air is (considered to be) constant; so the rate of diffusion is determined by oxygen saturation of the water at the air/water interface. In the case of still water the uppermost layer of water becomes quickly saturated with O2 and the diffusion of oxygen into the water slows down. For any given volume of water, the larger the surface area exposed to the air, the shorter the time required for the oxygen to diffuse to fully saturate the water column. This last statement is a very important consideration for both tank design (to maximise the surface area to volume ratio) and also in the active aeration from both water circulation and, potentially, filtration.

Photosynthesis
Oxygen is produced during photosynthesis. The simplified overall formula for photosynthesis is:

6CO2 + 6H2O + photons → C6H12O6 + 6O2    (carbon dioxide + water + light → glucose + oxygen),

As long as the energy available from the photosynthetically active radiation (PAR) exceeds the compensation point for photosynthesis, aquatic plants will be net contributors of oxygenation, to the extent that the water column may become fully saturated with oxygen, and the plants may “pearl”, with the O2 bubbles resultant from photosynthesis being out-gassed to the atmosphere.
Processes leading to de-oxygenation.
I’ll talk more about practical ways of increasing aeration in the tank, after I’ve listed the processes that reduce, or potentially reduce, the oxygen level of the tank water.

Respiration
All aerobic organisms respire. The reaction for the aerobic respiration is essentially the reverse of photosynthesis, except that now there is a large release of chemical energy (from stored
ATP molecules). The simplified version of this reaction is:

C6H12O6 + 6O2→ 6CO2 + 6H2O + 2880 kJ/mol
or
glucose + oxygen → carbon dioxide + water + energy.

Decomposition
The invisible biochemical processes of aerobic decomposition of organic material are usually described as the Biological or Biochemical Oxygen Demand (BOD), and the total biological demand for oxygen as the "bio-load." BOD represents the combined demand of all the aerobic metabolisms at work in the water, not merely that of the fishes and microscopic plankton, but also of the aerobic bacterial community. The metabolism involved in decomposition requires oxygen, often even more oxygen than respiration does. This is why water high in decaying organic material is characteristically also low in oxygen. Plants that aren't getting enough light to be actively photosynthesizing make their own contribution to BOD. It is easy to ignore the constant respiration of algae and plants, because photosynthesis produces such vast quantities of oxygen that it swamps the effects of cellular respiration during the day, but algae and plants are constantly respiring at the cellular level, night and day.

The decomposition reaction can be summarized as:

 Oxidizable material + bacteria + nutrient + O2 → CO2 + H2O + oxidized inorganic such as nitrate (NO3) or (Sulf(ph)ate) SO4.

Nitrification
One of the most significant oxygen utilising processes is in “Nitrification”, the biological oxidation of ammonia (NH3) to nitrite (NO2-) and then nitrate (NO3-).

NH3 + CO2 + 1.5 O2 NO2- + CO2 + 0.5 O2 → NO3-

Non-biological oxygen demand
The other contributors to de-oxygenation are the oxidizable inorganic compounds or “non-biodegradable oxidizable pollutants”. Not very informative as a title, but examples would be the effect of medications such as formalin (in “white-spot” medications) or the oxidation of metallic elemental iron to iron(III) oxide by oxygen, (more commonly known as “rusting” or “going rusty”.) 

4 Fe + 3O2 → 2 Fe2O3

The contribution of the inorganic compounds to de-oxygenation can be described as the “Chemical Oxygen Demand”, although this term is alternatively used for the laboratory technique(s) for estimating how polluted a water sample is, where both organic and inorganic pollutants in an acidified water sample, are fully oxidized to CO2 by a strong oxidizing agent (such as potassium permanganate KMnO4).


Fish, plecs and respiratory distress.
Another factor to bear in mind is that not all fish are born equal. Due to the effects of temperature on oxygen solubility, tropically warm waters are saturated with dissolved oxygen at much lower levels than cooler temperate waters. Fishes are evolutionarily adapted to certain oxygen levels and cannot be acclimatised to lower ones; for example a “ Hillstream loach” or L187a Chaetostoma milesi, which come from the cooler waters of rapidly-moving streams, will show respiratory distress long before a Gourami or Hypostomus that can supplement it’s blood oxygen by air gulping pdf filewill. (for a more technical description have a look at this excellent reference at “ Loaches online”.)

Carbon dioxide (CO2)
Somewhat more surprisingly fish breathing rapidly, or showing other symptoms of respiratory distress, are unlikely to be suffering directly from low oxygen levels, but are more likely to be stressed by high levels of carbon dioxide in the water. Without a sharp gradient between CO2 levels in the blood and CO2 levels in the water, it becomes increasingly difficult for the CO2 in the fishes blood to diffuse out across their gill surfaces into the water. As the water quality declines the BOD rises, and so does the CO2 content in the water. Consequently the fish often they rise and hang at the water surface gasping atmospheric oxygen. For Salmon and Trout dissolved CO2 in excess of 20 mg/L, can begin to impair the transport of oxygen (O2) in the fish's blood due to the Bohr effect, and it is likely that rheophilic plecs are similarly sensitive. Whenever fishes rise to the surface, an immediate partial water change is urgent, and additional water current at the surface is needed (to drive off CO2 into the atmosphere). Addition of an air stone might also help in such an emergency.

Nitrite (NO2-)
If fish are respiring heavily, it's also quite possible that they are suffering from nitrite poisoning, rather than from low oxygen levels. Nitrite in the water is drawn across the gill surfaces and occupies the sites on the blood's haemoglobin molecule that would ordinarily be transporting oxygen. This shouldn’t ever happen to your fish, because the symptoms of their distress should never become so extreme. If nitrite poisoning is suspected treatment with “Prime” or “Amquel +” would be the best option, although salt (NaCl) would be do in an emergency, if that is all that was available.

How to you know if your fish has a high oxygen demand?
My first port of call would be a reputable book or internet source, for example Ingo Seidel’s “ Back to Nature Guide to L-Catfishes” or the “PlanetCatfish” web site. If we know an exact collecting location, for example “Tapajos rapids”, we might expect that any fish from that location might be reasonably “rheophilic”, and be adapted to living in water with a high oxygen concentration. If we don’t have a collecting location we can use the morphology of the fish to give us some idea. I’ve used L235 Pseudolithoxus anthrax L235, as my “rheophilic fish template”.

Pseudolithoxus anthrax has:

  • An armoured body.
  • A flattened, stream-lined profile.
  • A large sucker mouth.
  • Big, stiff, pectoral fins.
  • Pectoral and pelvic fins originating from the side of the body, rather than underneath it.
  • A smooth ventral surface.

pseudolithoxus_ventral

Figure 1: L235 Pseudolithoxus anthrax - “White-spotted flyer cat” (images via PlanetCatfish)



pseudolithoxus_anno

Figure 2: View of smooth ventral surface, lateral fin insertion and enlarged sucker mouth.

The teardrop “tadpole” profile, flattened hydrodynamic body, stiff pectoral fins, dorsal odontodes and smooth ventral surface all help to generate down-force, as a film of fast flowing water is accelerated under the fish, sucking it on to the surface, where it attaches by the unique sucker mouth (which functions using the same principle). The armoured body helps to protect the fish during the inevitable collisions with rocks, and together with the body odontodes may also allow the fish to wedge itself in cracks and crevices when threatened, or when the river is in spate.

rapids

Figure 3: Typical rheophilic plec. habitat.

For those who are interested in a more complete description of rheophilic fish adaptations, I again recommend this article from the excellent “ Loaches Online”.

Amongst the other factors to consider is how much energy does your fish expend? Fish which show active foraging behaviour may require much more oxygen than those that have a relatively sedentary life-style. Plecs are probably mainly amongst the “couch potatoes” of the fish world, but an actively foraging Leporancthicus galaxias L007, may require more oxygen than a less mobile Panaque maccus L104.  An example of this effect is shown in Table 3; a study of oxygen consumption by the Rainbow Trout (Oncorhynchus mykiss). Rainbow Trout are an example of a fish which you could predict would have a high oxygen requirement; they are active, fast swimming fish native to clean, cool, upland rivers. All rheophilic fish are likely to show distress if dissolved oxygen (DO) levels fall below 5mg/l.

Table 3: The oxygen consumption (in g/kg/hour) of 100g rainbow trout (Oncorhynchus mykiss) as a function of water temperature and fish activity (the dissolved oxygen concentration in the water was 80% - 90%)

Water temperature (°C)

5

10

15

20

25

Reduced metabolism

20

22

72

90

138

Natural conditions without stress effects

100

180

250

280

-

Active metabolism (intensive feeding)

-

-

472

360

-

Forced swimming

-

480

580

544

478

Methods for increasing oxygenation
Having looked at the factors that reduce oxygenation, and also at those fish that are most likely to suffer from respiratory distress, it is time to consider what we can do about it. As many plecs come from very warm water, I’ll ignore temperature as a variable, other than to say that if fish from cool waters, they should not be kept in warmer water, as this is more likely to cause respiratory distress.

Oxygenation will be enhanced by:
  • Water circulation. As water circulates the uppermost oxygen saturated layer will be continually replaced by less oxygenated water from lower down the water column.
  • Good aquarium design. Ideally tanks should have as larger surface area as possible.
  • Filter design and hopefully its end result high water quality. Specifically this means maintaining low levels of nitrogen in the water.
  • Plants and substrate (but only in the appropriate context.)
  • Active aeration.

Water circulation.

I’ll look at water circulation first, in this case more really is better, and as much water turn over as possible is your aim. I should really have said as much flow speed as possible is your aim, as this is the critical measure for determining the rate of gas exchange. For those who are interested in the mechanics of aquarium flow, I suggest this link. Many of the links in this section will come from Marine Aquarists, as they are dealing with organisms that place particular demands on water quality. I’ll also say more under filter design, as good filter design can have the effect of vastly increasing the aeration of the water.

(image from the excellent “Loaches online”)

linear_flow”River tank” with linear flow.


Tank dimensions. How deep does your tank need to be? A wide shallow 500 litre tank of 6’ long x 3’ wide x 1’ tall (approx 180cm x 90cm x 30cm, surface area 16,200 cm2) would have a surface area twice as large as one of the same capacity measuring 6’ long x 18” wide x 2’ tall (surface area = 8,100cm2).

Filter design.This really is the most important factor in maintaining good aquarium oxygenation. Many aquarists assume that buying an ever bigger filter will compensate for other factors, as well as looking for a filter which will combine the roles of filtration and water circulation, which although they may be related are not the same thing.
I’m going to ignore both chemical and physical filtration, other than to say that the physical removal of faeces and saw-dust by the filter is not necessarily a good idea, and these may be best removed by siphoning, or other method. The faeces of an herbivorous fish, or the saw dust produced by a wood eating “xylophagousPanaque, are unsightly but they are not actually very polluting.

The 2 characteristics of the filter that are of primary interest to tank aeration are:
  • Biological filtration capacity.
  • Gas exchange capacity.

The biological filtration capacity is important because often the variable that is having the largest effect on oxygenation is the “biological (or biochemical) oxygen demand” (BOD) or "bio-load." BOD represents the combined demand of all the aerobic metabolisms at work in the water, not merely that of the fishes, invertebrates and microscopic plankton, but also of the aerobic bacterial community. Actually calculating the BOD is a difficult outside of the laboratory, but the concept is important because it's all too easy to underestimate the bio-load, in merely correlating it with the weight of fish in the aquarium. The invisible processes of aerobic decomposition of organic material are also part of the BOD.

BOD, oxygen demand and the nitrogen cycle
Organic matter is literally broken down by fungi and aerobic decomposing bacteria utilising oxygen and eventually producing ammonia. These ordinary “heterotrophic” bacteria are opportunistic scavengers, reproducing rapidly every 15-20 minutes. The more waste is in the aquarium, the larger the heterotrophic bacterial colonies will grow, and potentially the more oxygen they will consume. The contribution of organic matter to the BOD depends upon its composition, a food source consisting of structural carbohydrates such as the cellulose and lignin (wood for example) will be decomposed very slowly and contribute little to the BOD, whilst soluble protein and/or sugar rich substance like sweet potato or carnivore pellets will contribute greatly.

The nitrogen cycle
The other source of ammonia (NH3), and contributor to BOD, is as a by-product of all aerobic metabolisms, excreted through the fish’s gills and by snails, copepods, fungi, bacteria etc. Ammonia is converted to nitrite and nitrate in the nitrogen cycle, and these nitrifying bacteria require much more oxygen than the "ordinary" metabolism of aerobic bacteria, the ones that break down organic matter and metabolize organic carbon.

  • The important factor to remember is that if BOD is reduced, nitrogen conversion is enhanced.

The efficiency of the nitrifying bacterial metabolism depends on a large water surfaces exposed to oxygen (the “Gas Exchange Capacity”), this is because the nitrifiers compete poorly for oxygen with the community of bacteria that are breaking down the organic matter, the ones responsible for much of the BOD. Simply stated, a heavy load of organic materials being degraded in your system inhibits the nitrifiers by competing with them for oxygen. Dissolved oxygen concentrations above 1 mg/l are essential for nitrification to occur. If DO levels in the filter drop below this level nitrifications slows, or ceases altogether, with potentially catastrophic effects.

Biological filtration capacity, gas exchange capacity and filter design.
When we bear all these factors in mind, it becomes apparent that the volume or capacity of a filter may not actually be that important. For example an external power filter containing a large volume of ceramic media or sintered glass, and with high water turnover volume (x 10 or more), may be working at a fraction of its capacity, if the water is rapidly de-oxygenated during its initial contact with the filter media. A larger volume filter will add more potential sites for biological filtration, but if the factor that is limiting nitrification is the oxygen supply, they will remain as potential, rather than actual sites. A filter which is extracting a large amount of faeces and saw-dust may become partially clogged, reducing flow and also essentially oxygen, with a “double whammy” as the bacteria degrading the organic materials now inhibit the nitrifying bacteria in the filter by competing with them for oxygen.


trickle

A schematic cross-section of the contact face of the bed media in a trickling filter

 

This is also why “bio-wheels” and “wet and dry” trickle filters, with large gas exchange capacity, are so effective at both nitrification and oxygenation; they have filtration media with a thin film of flowing water allowing rapid diffusion of atmospheric oxygen to the nitrifying bacteria, and if left relatively undisturbed a more complex biofilm of aerobic and anaerobic bacteria may develop.

 

From: http://en.wikipedia.org/wiki/Trickling_filter

 

Carbon dioxide – CO2.
As signs of respiratory distress are often related to high CO2 levels in the water, we should definitely not add CO2 to tanks containing plecs, in fact the 20ppm CO2 level suggested for improved plant growth is about 40 times the natural atmospheric content. If we have efficient gas exchange, either by water movement or filter design, as well as oxygen dissolving into the water, CO2 will dissolve out until it equilibrates with atmospheric CO2 levels (about 0.03% of the atmosphere). In contrast to what is often written, unless we are physically adding CO2 to the water, water movement, and trickle filters, will add CO2 to the water when plants are actively photosynthesising (utilising CO2 and producing oxygen), and out-gas CO2 when levels in the water exceed those in the atmosphere. Typical CO2 levels in tank water are dependent upon temperature, pressure and the carbonate content of the water, but should be in the range of 1 – 2ppm (at sea level, at 27oC, pure H2O is saturated with 7.9 mg / l O2, but with only 0.42 mg / l CO2).

Plants, structures and substrate.
Aquatic plants will be net contributors of oxygenation when they are photosynthesising, but when they are not they will be part of the bio-load. You could take advantage of 24 hour photosynthesis by using a sump refugium with reversed lighting regimes (dark in the tank, light in the refugium). However in any aquarium, with actively growing plants, their oxygen production will massively exceed their oxygen usage, and realistically oxygen levels will be similar, or higher, at night in the planted tank when compared to an unplanted tank. This seems a nonsensical statement, but particularly for emergent plants (with access to atmospheric oxygen and CO2), the plant may be able to use atmospheric oxygen for respiration, even in roots deep in the substrate. This is a quote from the abstract of Li & Jones 1995 paper entitled 'CO2 and O2 transport in the aerenchyma of Cyperus papyrus L.'.

 “…..While the water surrounding the rhizomes remained strongly hypoxic, the O2 concentration in the submerged rhizomes was 15.1% during the day and 10.3% at night.….”

Another advantage is that ammonia, nitrite and nitrate will be preferentially taken up by the plants, and the ammonia they utilise will not enter the bacterial nitrogen cycle, and therefore will not deplete the tank oxygen in the same way that it would if plants were absent. A thick layer of floating plants (Caraco & Cole, 2002) will potentially have a greater effect in reducing dissolved oxygen (D.O.) levels, but this is not likely to occur in tanks with high flow speeds.

Structure and substrate
Might seem as a bit of a strange section in a page on aquarium aeration, but as well as the filter, plants, wood and other surfaces in the aquarium offer a potential home to the community of aerobic bacterial that metabolize ammonia to nitrite and then nitrate. The uppermost surfaces of the substrate are a good location for these bacteria, because the nitrification process uses a lot of oxygen. However only a few centimetres below the substrates’ surface, the diffusion of oxygen can't supply enough oxygen, and as oxygen levels fall anaerobic bacteria become more frequent (in exactly the same way that is shown in the schematic drawing of a cross section of a trickle filter). Many of these bacteria are in fact “facultative anaerobes”; when oxygen is in short supply, they are able to switch to a metabolism that doesn't require oxygen, instead, they use nitrate, stripping the oxygen and leaving nitrogen (N2) gas. The nitrifying bacteria provide the nitrate, and their high oxygen demands also tend to exhaust the limited supply of oxygen. These two types of bacteria will occur across a fluctuating boundary lying not far beneath the surface of the substrate. The same processes will also occur in the “rhizosphere” the aerated zone lying around aquatic plants roots. These processes are both a good reason for:

  • having a substrate, and
  • leaving it relatively undisturbed.

There is a lot more about the microbial dynamics of sediments and how they interact with oxygen and nitrogenous compounds on the “Skeptical Aquarist”, and also on this page from Edinburgh University.

Direct aeration
Direct aeration is the process where atmospheric oxygen is introduced into the aquarium, it can be either by:
  • hydraulic aeration, or by
  • air diffusion


In hydraulic aeration, for example by a spray bar, the water jet comes out from the nozzle with high velocity and falls into the water in drops, thus contacting the air on a large surface and dissolving oxygen from it, additionally when the water droplets hit the water surface turbulence occurs increasing the diffusion of air into the water, and CO2 into the atmosphere. Alternatively the water may be forced through a venturi-type diffuser, where its pressure decreases below the atmospheric pressure and air is sucked into the water. Both techniques have been widely used in waste water treatment.

In diffusion aeration the air is supplied by an air pump, and introduced into the aquarium through a diffuser. In this case as well as diffusion from the bubbles, when the air bubbles hit the water surface turbulence occurs, again increasing the diffusion of oxygen into the water at the water surface.

The more water flow and/or bubbles there are, the more surface turbulence there is. The bubble size is also important, in that the smaller the bubble the greater its relative surface area is, and the more diffusion of gas into the water will occur. What you do have to remember is that air is only 21% oxygen, and oxygen is much less soluble in water than, for example CO2. This means that an air pump needs to produce very fine bubbles (in the range of 10 – 200 microns diameter), that have a long “residence time” in the water column, if significant exchange of oxygen to the water is to occur. For maximum residence time and effect, unless you have a “wet and dry” or bio-wheel filter it would be advantageous if the filter intake picks up both the bubbles and oxygenated water, and feeds them straight into the filter where they will provide much needed oxygen to the nitrification process.

Diffusers and bubbles
Ceramic flat plate diffusers are one possibility for producing fine bubbles, and they are widely used in aquaculture, they have the disadvantages of being expensive, requiring high air pressure and clogging relatively easily. The other option is a membrane diffuser, which has the advantages of not clogging so easily and requiring lower air pressure. The technical term to look for is “EPDM“ (ethylene propylene diene terpolymer), and the membrane diffuser can be in the form of  a disc, tube or “air wall”. These kinds of diffuser are widely used in the waste water industry, and in aquaculture, where the BOD of the water may be several hundred times the waters natural oxygen holding capacity.

The answer
The answer is there isn’t really a single answer, but there are certain things you can do to help maintain oxygenation.

  1. Keep up your water changes, with the aim of reducing nutrient build up.
  2. Accumulated nutrients are a direct result of:
    • overstocking an aquarium
    • overfeeding the system,
    • inadequate filtration, and / or insufficient filter maintenance.

  1. A high water flow rate increases gas exchange - CO2 out, oxygen (O2) in.
  2. Biological filtration capacity is often limited by lack of oxygen, not lack of filter material, bigger is only better if sufficient oxygen is reaching all the of the aerobic bacteria in the filter.
  3. A large capacity wet/dry trickle filter, where the thin film of slow flowing water will become fully oxygenated, is likely to produce much more highly oxygenated water than most other types of filter. A back up for both biological filtration and aeration is essential, battery powered air pumps and sponge filters can be the difference between life and death.
  4. To fully utilise the filtration capacity of a large volume canister filter, water flow, and the oxygenation levels of that water, must remain high.
  5. Bulky organic debris (food, faeces, saw-dust) can reduce the filter capacity for nitrification and aeration, both by the oxygen consumed during decomposition, and by reducing the flow speed, and water volume, passing through the filter.
  6. Plants and substrate can contribute to maintaining water quality, and therefore oxygenation.
  7. Aeration, using an air pump and diffuser, can increase oxygenation, particularly if the bubbles are very fine, and have a long “residence time” in the water column or filter.
  8. “More fish” are almost certainly too many.

References (these links are just a selection of those that are available)
Web



wikipedia

Scientific Papers

  • Armbruster J. (1998) “Modifications of the Digestive Tract for Holding Air in Loricariid and Scoloplacid Catfishes” Copeia 3 pp. 663–675.
    Summary
    Loricariid catfishes have evolved several modifications of the digestive tract that appear to function as accessory respiratory organs or hydrostatic organs. Adaptations include an enlarged stomach in Pterygoplichthys, Liposarcus, Glyptoperichthys, Hemiancistrus annectens, Hemiancistrus maracaiboensis, Hypostomus panamensis, and Lithoxus; a U-shaped diverticulum in Rhinelepis, Pseudorinelepis, Pogonopoma, and Pogonopomoides; and a ringlike diverticulum in Otocinclus. Scoloplacids, closely related to loricariids, have enlarged, clear, air-filled stomachs similar to that of Lithoxus. The ability to breathe air in Otocinclus was confirmed; the ability of Lithoxus and Scoloplax to breathe air is inferred from morphology. The diverticula of Pogonopomoides and Pogonopoma are similar to swim bladders and may be used as hydrostatic organs. The various modifications of the stomach probably represent characters that define monophyletic clades. The ovaries of Lithoxus were also examined and were shown to have very few (15–17) mature eggs that were large (1.6–2.2 mm) for the small size of the fish (38.6–41.4 mm SL).
  • Caraco, N. & Cole, J. (2002) “CONTRASTING IMPACTS OF A NATIVE AND ALIEN MACROPHYTE ON DISSOLVED OXYGEN IN A LARGE RIVER.” Ecological Applications 12:5 pp. 1496-1509. Summary
    In aquatic systems low dissolved oxygen (DO) has been identified as a serious water quality problem. Here we use empirical data and modeling to explore the hypothesis that the introduction of an alien aquatic macrophyte (Trapa natans) may have had dramatic impacts on the frequency and extent of low DO events in the Hudson River. Continuous measurements with moored instruments demonstrated that in large macrophyte beds dominated by a native species (Vallisneria americana) DO never declined below 5 mg/L during the summer growing season. In contrast, during this same time period, extremely low DO was common in large beds dominated by Trapa natans, with DO values below 2.5 mg/L occurring up to 40% of the time. This difference in DO can be modeled based on species differences in the balance of respiration and in-water photosynthesis. The low DO values in Trapa beds suggest that these beds may be poor habitats for sensitive fish and invertebrates and that redox sensitive chemical reactions may be altered within Trapa beds.
  • Carlson R. & Lauder G. (2009)
    “Living on the bottom: Kinematics of benthic station-holding in darter fishes (Percidae: Etheostomatinae)” Journal of Morphology (in print) http://www3.interscience.wiley.com/journal/122515452/abstract
    Summary
    We found that on both substrates, the two species generally exhibited similar kinematic responses to increasing flow: the head was lowered and angled downward, the back became more arched, and the median and caudal fin rays contracted as water flow speed increased. The ventral halves of the pectoral fins were also expanded and the dorsal halves contracted. These changes in posture and fin position likely increase negative lift forces thereby increasing substrate contact forces and reducing the probability of downstream slip.
  • Garavello, J.C. & Garavello, J.P. (2004)“Spatial distribution and interaction of four species of the catfish genus Hypostomus Lacépède with bottom of Rio São Francisco, Canindé do São Francisco, Sergipe, Brazil (Pisces, Loricariidae, Hypostominae).”
    Brazilian Journal of Biology 64:3 pp.103-141.
    Summary
    The large catfishes, genus Hypostomus Lacépède, are common species in almost all freshwater environments of South America. The behavior of specimens from the species Hypostomus alatus, Hypostomus francisci, Hypostomus cf. wuchereri, and Hypostomus sp., from the region downstream from the Xingó Hydroelectric Power Dam, located in Canindé do São Francisco on the Rio São Francisco, were observed. Morphological characters collected from preserved specimens were also studied in the laboratory, in order to shed light on interactions between those fishes and the river bottom. In addition, the formulation of Gatz (1979) was applied to better explain the environmental interactions of Hypostomus species. This study revealed that the fishes utilize their suckers, through a mouth equipped with an oral disk, as well as the pectoral, pelvic, and caudal fins areas, to interact with the rocky river-bottom.
  • Li, M. & Jones, M. (1995) “CO2 and O2 transport in the aerenchyma of Cyperus papyrus L." Aquatic Botany 52, pp. 93-106
    Summary
    Cyperus papyrus L. (papyrus) is an emergent wetland species with C4 photosynthesis. Culms of papyrus possess numerous large intercellular air cavities and functional ‘Kranz’ chlorenchyma which are involved in CO2 recycling in the culm. In darkness, the CO2 concentration in the culms increased to 74 times that of the ambient air. In the light, the culms greatly reduce the intercellular CO2 concentrations by internal CO2 recycling via photosynthesis. Results suggest that 35–57% of the CO2 respired by the culm pith and rhizomes may be refixed by culm photosynthesis. The dynamics of O2 transport in the intercellular spaces of the culms and the rhizomes were also studied. Both illumination and prolonged darkness had significant effects on the O2 concentrations in the culm and rhizomes. While the water surrounding the rhizomes remained strongly hypoxic, the O2 concentration in the submerged rhizomes was 15.1% during the day and 10.3% at night. The diffusive fluxes of CO2 and O2 within the papyrus plant during the day and night were calculated. Results suggest that rapid CO2 exchange occurs between the ambient air, internal atmosphere and the culm photosynthetic tissue. Also, there is a high O2 flux, particularly at night, which is generated in the intercellular air spaces between the culm and the rhizome.
  • Power, M. (1999) “ Life Cycles, Limiting Factors, and Behavioral Ecology of Four Loricariid Catfishes in a Panamanian Stream pdf” In G. Arratia, B.G. Kapoor, M. Chardon, & R. Diogio. (eds.) “Catfishes”. (2002) Vol. II.
    Summary
    “In this chapter, I summarize field observations on the behaviour and natural history of loricariids near the northern boundary of the family’s natural distribution in the Rio Frijoles (9'9' N, 79'44'W). This streamdrains secondary tropical rain forest of the Parque National Soberania in central Panama. The Rio Frijoles was once a tributary of the Chagres River but after construction of the Panama Canal, now empties into Lake Gatun. Four loricariids occur in streams of the Parque National Soberania: Ancistrus spinosus (Eigenmann et Eigenmann), Hypostomus (Plecostomus) plecostornus (Linnaeus), Rineloricurius (Loricaria) urucunathus Kner et Steindachner, and Chaetostornus fischeri Steindachner. Most of my observations are on Ancistrus spinosus, the most common loricariid in deeper stream pools of the Rio Frijoles.”