I've got a 45G freshwater planted tank that has been in place since May, fully cycled, lightly stocked. I had two red serpae tetra in there from the beginning, later made it 5, and added 5 congo tetra and two un-mated angels, plus a cleanup crew of 10 or so ghost shrimp, a raphael catfish and one BN pleco, plus a few snails.
The congos were such aggressive eaters, I moved them out and left the five serpae tetra for even lighter stock. The angels are not aggressive to the tetra.
About 2-3 months ago one serpae stopped eating, and wasted away very slowly over a two month period.
Now another is starting to do that. There's no obvious symptoms other than not eating, and starting to look a bit ragged. Even at the end the other just looked a bit pale and thin; the difference in appearance is somewhat subtle. They also look like they have more trouble swimming; toward the end the other was nose up most of the time, this one is level but each pump of the fins is more pronounced like it takes more effort.
I really need to emphasize that the change is very slow, and quite subtle; you just realize after a while one is not eating anything, always the same one (as it gets thinner and more pale that is easy to tell).
None of the tetras eat well. They'll take bloodworms readily, but flakes and pellets they are as likely to ignore as eat. One (poor quality) flake they do a bit better with, but the weak one (now second one) won't even eat it.
The angels and indeed everything else in the tank are quite healthy, in fact the angels have at least doubled in size in the last 3-4 months. The remaining 3 tetras look and act healthy, they are just eating very little. I would not worry about it other than one dead and one getting worse.
I THINK all four remaining tetras are male. There is not much aggression even so, though they all do take turns chasing each other (well, the current weak one is chased, haven't seen him chasing). But not much, and while the fins are a bit ragged I do not think aggression is the issue. But since I am unsure what the issue is...
There is no sign of a parasite visible, no ich, nothing obvious or moving in their poop. The dead one, when I removed it I examined closely, had no sign of anything internal (as best you can tell from an up-close external look - but it was pretty thin by then); I think I would notice some of the worm like parasites.
There are no sores, no flashing going on, no noticable difference in gill color, no difference in eyes (other than as they get thin they look bulging a bit).
I'm posting a shot of the tank for overall perspective, and a shot of a relatively healthy one with the current weak one in the background. The weak one is more lit by the flash, but the somewhat pale look to the scales is real. Also, there's a difference in the shape of the eyes and nose that is real also, that I attribute to being thinner. The one that did had this even more pronounced, with eyes bulging a bit near death.
I'm somewhat inclined to just decide there's something inhospitable about this tank for serpae, and replace them with something else, maybe just pair off the angles (I have some more in a large tank and can find a mate for one of these, I'm sure).
But I'm also curious if anyone has any idea what might be causing this? I hate mysteries, and have been hesitant to add more serpae (which might be generally healthier) for fear there's some disease in there specific to them.
45 Gallon, ecco-complete + gravel substrate, planted as seen, moderate flow (SunSun 404B, spraybar at left as seen).
78F, ph 7.8, dKH
6, dGH 9, nitrates run between 10-20, iron trace, phosphates 0.05 or so (trying to raise it), no CO2
, daily 5ml excel, 5ml iron, 2ml Phosphorous, weekly flourish 5ml comprehensive, weekly 5ml potassium.
Water is RODI
water remineralied with Equilibrium, acid + alkaline buffer aiming at 7.0, but the ph still stays 7.6-7.7.
Feeding tetra flakes, occasional frozen bloodworms or shrimp, trying new life small fish pellets (no luck with the tetras, the angels eat anything).
Light is on for two 4 hour periods daily, plants are growing well.