I woke up this morning to find my 2 tinfoil barbs dead floating at the top of the tank, and my piranha upside down breathing heavily at the bottom. I did a water test, the only thing that was off was the Nitrates level which was about 60ppm. Thing is the Oscar, Cigar shark and 2 plecos in the tank are all fine. I have since done a mass water change on the tank and have moved the piranha into a 66 temporarily but her condition hasn't changed. Is this a case of nitrate poisoning or what? If so, do you think she'll make it or should I just put her down?