Hi all,
I'm not going to get involved in the argument as such, but I'd agree with the other posters about the unsuitability of plecs for Malawi tanks. I know
Synodontis catfish don't do the same job, but they do come from the rift lakes and are better adapted to the water, particularly the Lake Malawi native
S. njassae. Details here: <
http://malawicichlids.com/mw11003a.htm>.
There is a real scientific reason for not putting soft water fish in hard water.
First up Lake Malawi is fairly carbonate rich (high kH), and well buffered, so the water is very different to any in S. America, where even the white water rivers are carbonate poor (low KH).
You can get a breakdown of the water for the Rift Lakes which shows why you shouldn't keep Lakes Tanganyika cichlids with Malawis, <
http://malawicichlids.com/mw01011.htm> and just how alkaline Lake Tanganyika is.
I was quite interested in this as well, L273 wrote:
My pH spiked from 6.8 to 7.5 and i lost quite a few small L128's a couple of months back now.
First of all sorry for your loss, but I don't think pH was the only issue here, or possibly not even an issue at all. The reason I say this is that pH is a bit of a funny measurement, it is the ratio of acid:alkaline ions, expressed as the "
negative log of the H+ ion concentration" not a very useful definition, but what it means is that at neutral pH7 you have the equivalent of equal numbers of H+ (hydrogen ion) & OH- (hydroxide ion) (from the dissolution of H2O = pure water).
It is also a LOGbase10 scale meaning that pH6 has 10X more H+ ions than pH7, but 100X more than pH8 and 1000X more than pH9.
Because of this log scale the difference from pH6.8 to pH7.5, even though we have gone from "acid" to "alkaline", is relatively small, less than x10 fewer H+ ions, and much less than say going from pH8.2 to pH8.8. and as such it is very unlikely to have caused the fish enough of a problem to kill them.
In water with little (bi)carbonate buffering ((HCO3-) CO3-) the pH can fluctuate really wildly. This is why people who add CO2 (which will go into solution as "carbonic acid" - H2CO3) to their tanks, buffer the water up to 4dKH.
The factor relevant to plecs is that even if S. American rivers (like the upper Amazon - Rio Ucayáli) that have pH above pH7, they have virtually no carbonate buffering (dKH) and that pH can fluctuate.
As I said earlier this is because we are talking about both a ratio (pH), and also amount of solute (ions or buffering). In the Rio Ucayali situation we have a 1:1 ratio (pH7), but low levels of dissolved ions (measured as the TDS or electrical conductivity). This means as soon as we add any acid producing substances (tannins from dead leaves, humic acids from organic matter) the pH will drop rapidly.
In the case of the carbonate buffered water we might still have a 1:1 ratio of ions and pH of pH7 (if there are lots of both acid and alkali ions and a high TDS) but it will take a large addition of acid to make the pH drop. This is the important bit, fish are actually much less sensitive to pH than we think. What causes the problems are the amount of solutes and their nature, which we see reflected as the pH. The Lake Malawi tank isn't unsuitable for any plec because it has a high pH, it is unsuitable because the water is full of alkaline ions. We could balance those ions by adding an acidic salt like sodium bisulphate (NaHSO4), this would lower the pH, but the water would have even more salts (as ions) than it did before and would be even more unsuitable for fish originating from soft water.
cheers Darrel