Mysterious Cory deaths
I have an issue with my corys which started a while ago now. First noticed it when one of the Cory's was acting strange - lethargic at first and resting in odd places, then rapid breathing and laying at strange angles/ on its side. After a few days this Cory died. There were no physical signs of illness at all. A few days later another Cory started acting the same way and died the next day. This has now killed 4 Cory's, one by one. All other fish in the tank seem completely fine, and the other Cory's at the moment seem completely fine.
Tank is 50 gallons and contains 8 cardinal tetras, 7 ember tetras, 1 BN pleco and 3 amano shrimp. There were 7 delphax corys, now 3.
Water seems fine, 0 ammonia and 0 nitrite and 15 Nitrate. 24-25 degrees Celsius. Cory's are fed a mixture of wafers, bloodworm and pellets and seem to eat OK. Most of the Cory's that died were 2-5 years old and seemed healthy until recently.
Any idea what might be causing this and what I can do?