Hi everyone. I have a freshwater 75 gallon aquarium. I originally had 5 koi angelfish, a blue gourami and three Cory Juliis. I had them for about a month/month and a half. Not too long ago I experienced a ammonia spike. The ammonia spike ended up killing two of my angelfish. I immediately started doing water changes and everything is going back to normal. However, I noticed a new problem. I noticed black spots on my angelfish. I was told that it was normal, and that it’s black spot disease. I was recommended Bifuran+ to treat my tank. Within three days, I didn’t notice a change and during performing water changes to get the treatment out, two of my Corys passed. I don’t know what’s going on here. My koi angelfish are very skittish, rub themselves against the glass, and the one gasps for air. The other Cory is active and seems to being eating. My blue gourami is eating and seems okay. My Ph: 7.5, Gh: 30. Nitrate: 0. Nitrite: 3, it’s still going down. Can anyone help me identify what’s happening. Is it gill flukes, another parasite?