You're too vague for anything to be found out. You'd have to list details, changes to environment (decorations (copper is like poison for fish, did you maybe drop pennies in the tank to look pretty?), new flooring, different cleaners for the tank), what kind of fish they even were in the first place, the type of tank they were in, the type of care they received and foods they were fed.
For all I know a giant sunbeam of fish killing magic hit them and they were fried dead on the spot.
Please provide more information so some help can be offered up