I have a 55 gal tank. I use 2 filters. 1 rated to 40 gal on one end of the tank the other rated to 30 gal on the other end. The heater keeps the temp at 78. 2 air-pumps with 2 18" air-stones. I check the nitrites and Ph regularly and keep up with my water changes including gravel cleaning. In the past 2 weeks I have lost 7 or so fish. They all seem healthy one minute, completely dead the next. No odd swimming, funny colors, missing scales, shallow or deep breathing or flashing. I use tap water to which I add aqua-safe and neutral regulator. I add extra carbon to the 40 gal filter as it is a whisper with an open-able cartridge. I lost 3 molly's, 3 painted glass, 1 gold guarami, 1 beautiful black angel( my buddy), a 7 year old large pleco. Still living is an angel that came down with what i think is dropsy 3 days ago that i cant afford to treat, 3 red molly's, 1 emerald cory, 1 clown loach, 1 large pleco I got to replace my old one, 1 random catfish and a 6 or 7 inch black ghost knifefish I intend to move to a larger tank once he starts gettin bigger. The knifefish is the only aggressive fish and he keeps to himself unless someone comes in his tube which they don't. I pretty much gave up hope after my black angel died. He used to follow me around the room and if I reached in the tank for anything he would brush up against me. He was big and healthy and to see him happy one minute and dead the next made the whole situation seem kinda hopeless. Any thoughts?