I'm sorry to hear this
I reckon you need to start from scratch. Somethings gone down and if it were me, I'd redo the whole tank. Move all the occupants into another tank or big bucket with the heater and filter and drain the aqaurium. Wash the gravels, rocks, ornaments, etc really well and replace them again. Fill it back up and after it's stabalized, acclimitize the fish back in. This should help. Then again, it's just my opinion 8)