Sorry about your lost. Is there anyway that you added anything to the tank that may have been comtaminated? Couple years back I was soaking some driftwood in my back yard and I threw it in my tank. The next day all my dats were dead. My family told me that they smelt poison. The neighbor behind my house is a prick and my pops caught him spraying poison up towards our house. Guess it got on the driftwood and killed my fish. Took me awhile to figure it out.