05-10-2009, 07:45 PM
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It's sometimes difficult to diagnose why a fish dies, but I will share a couple of observations. I have maintained several of the pencilfish species over the years. Common names are not always reliable, so I'm not sure if you have the common dwarf pencilfish Nannostomus marginatus, or the coral red dwarf pencilfish Nannostomus mortenthaleri, or even some other fish. I've tracked down a couple of photos and attached them; the left photo is N. marginatus and the right photo is N. mortenthaleri.
Most of the pencilfishes are wild caught and they are not the easiest to adapt to water that is significantly different from their native habitat. The dwarf pencilfish, species Nannostomus marginatus, comes from several sites in the Amazon basin, notably the Rio Negro and its tributaries. The water in the Rio Negro is very soft (there is basically no mineral content to speak of) and has a pH of 3.4 to 5.5 depending upon localities. The N. mortenthaleri comes from forest tributaries of the Rio Nanay in northern Peru. I would expect the water to be acidic, although probably not as much so as the Rio Negro. Each degree in the pH scale (from 5 to 6 for example) represents a ten-fold change in acidity, so a pH of 6 is ten times more acidic that pH 7.0, and pH 5 is 100 times more acidic that pH 7.0, etc. This is quite a variance from your tank's pH of 7.5 and I suspect your water is on the hard rather than soft side in its mineral content. Pencilfish are sensitive to water chemistry and do not always adapt easily to water that is vastly different. I've mentioned in other threads the reason for this.
Another possible problem could be the aggressiveness of males particularly with N. mortenthaleri. I have not managed to find any of this beautiful species myself, but in an account of this fish in Practical Fishkeeping, Dr Karel Zahradka reports that in a restricted space (too small tanks) the males will kill each other. I have read elsewhere that this occurs with many of the pencilfish. Did you notice any aggression between the two fish you had? The lack of colour and non-eating are sometimes indications that the fish is being "bullied."
A related point is that pencilfish are characins, and like tetras (also characins) they are shoaling fish that prefer to be kept in groups. There are several tetras where the males will become more aggressive if they are kept 2 or 3, whereas in groups of 6 or more there is less direct aggression. But this of course requires a larger tank.
The above are a couple of reasons, and of course it is always possible that the fish carried some internal parasite or abnormality. But for the future, I would suggest providing pencilfish with water closer to their natural environment, slightly acidic (pH 6.5-7.0) and in small groups.