I have lost 3 panda Cory in 5 days. Two today. Nothing visibly wrong. Parameters are 0, 0, 20. Just did pwc and nitrates now 5ppm. PH is 7.8. Can’t find test kit but GH 10 and KH 6 or 107 last test. Temp 76F. Nothing added to tank except dechlorinator. Fish treated with General Cure over a week ago for possible internal parasites. No other fish effected. I have 26 more cories and 49 Neon tetras. 54G tank with Aqua Clear 70 and 59G sponge filter. Any ideas?