I've had a number of cories, some albino, some bronze (both aeneus) and some peppered.
Of the bronze and albino, I have lost only 1. I have lost 4 out of 6 of the peppered. They lasted a while and all went at different times, but in the exact same way. They would float and appear very limp. Initially I thought they were dead but their gills would move slightly. They would do this for periods of time and then swim about again seemingly happy and healthy enough. This would last a period of 2-4 days depending on the fish and eventually they would die. There were no other signs of illness.
At these times I checked the water parameters and all were OK.
Now the same is happening to the 5th peppered cory. I checked my water parameters earlier tonight when I noticed this behaviour:
Nitrate 20 (higher than normal, usually it is 10)
I wouldn't say I'm worried about the nitrate because it's still not at a poisonous level, and since this is the 5th time this has happened, and only the first time in this situation that the nitrates have exceeded 10 I don't think it is the culprit.
From the first death to now, approx 6 or 7 months have passed.
Does anyone know why this is happening, and why to only the peppered? Are they more delicate than the others? From what I've heard they are a hardy species of cory. I'm just at a bit of a loss as to why they are, one by one, just wasting away.