You best route is to go for a HMA unit and a RO unit
This enables you to mix your water to suite the species.
I will try to explain HMA and RO
HMA is a means of removing heavy metals and solid matter.
First we have to understand water has many things that get added to it over the course of supply.
At first the water companies add various chemicals to ensure they meet the legal requirements. The main items are chloride fluoride and phosphate. All have a sound reason behind being added.
Next on its journey to your tap water “Picks up” various salts, metals and chemicals from the pipe work and various items it will come in to contact with.
Naturally none of these are present in a natural river system that is not subjected to farming or industries.
HMA will remove a very high amount of metals and semi solid matter RO will remove most chemicals and items that create a hazarded for fish.
Each type of filter works in basically the same way a pre filter will remove the larger items like sand dirt ect next in a RO unit a carbon filter will remove the tastes and stable pollutants, along with chlorine which will damage the RO units membrane should it come in to contact with it. Finally in the RO unit a high quality filter working on the pressure principal will allow a small amount of water to pass through the membrane, this water will be around 30% of the total water passing over the membrane. This is why you get so called waste water from a RO unit, it is not waste as in it contains the extra “Gunk” the unit has removed it is just water that cant pass through the membrane.
You will hear RO Units described as 3/ 4 / 5 or 6 stage units this is the amount of filter stages that are used. A 3 stage is a stranded type some time people use a 4 stage unit the 4th stage is normally a resin exchange unit to give the water a final “polish.”
The HMA unit works differently in that it used a very highly treated second stage to remove the final metals ECT, in fact HMA filters are some time called “Heavy Metal Axe “ filters. You will also hear them called CBR filters.
The HMA filter will not alter the TDS (totally dissolved solids) or the PH.
I use a very fine needle valve to mix the two. For myself I control the amount of HMA I add thus the water will always start os pure RO.
I can increase the amount of HMA to give me a higher TDS and use this to stimulate spawning or aid other behaviour I wish to bring on.
Words of warning
RO is totally unstable due to having no carbon hardness so it will follow the path of least resistance that is to say it will buffer or drop depending on its environment, not what you want in a fish tank.
The mixing of “ mineralised salts” is usually the way most fish keepers control this instability.
I use a very small amount of mineralised salts as I am adding HMA water.
To fully understand how RO water can be used you must first understand the way water supports life and the general behaviour of water.
I spent around 6 years getting it right and still have a few areas I don’t fully understand.
Should you ever want to visit Bristol you are more than welcome to pop in and see how I do it.
http://en.wikipedia.org/wiki/Reverse_osmosis
