Today I discovered my one of my panda cories, the lone male, dead as I was doing a water change. Now I am assuming he was male, because he was considerably less-plump than all the others, and his blackness was more prominent. His death didn't surprise me too much, though, because he had been behaving abnormally for the past week or less. He would breathe more rapidly than the other 5 cories, and forage less frequently for food, and he would not hide in the shade of the driftwood/plants. Looking at his body after he died, he had some light red splotches near his belly. His unusual behavior and the splotches are the only evidence I have towards the unknown cause of his death. Any ideas? I'd like to keep the other cories and fish as healthy as possible.
The tank is a 55 gallon.
Lighting: 2 18Watt fluorescent on for 9 hours
Water parameters: ammonia=0, nitrite=0, nitrate=40, pH=7.5
Other fish: 5 black skirt tetras, 1 pearl gourami, 1 bristlenose pleco, 1 mystery snail, 5 panda cories (living), 1 black molly
Filtration: 2 HOB (used) equivalent to Penguin 330
Last water change & amount: Today, 25%
Feeding: Flakes daily, crisps and freeze dried bloodworms occasionally; shrimp pellets and veggie wafers alternated daily