Any water parameter issues, and what kind of fish? A good way to check things would be to get an extremely hardy fish like a damsel now. See how it does over a month or so. (they make good sump monsters once you're done, btw)
Have you seen any signs of ich? Is so, you'll need to leave the tank barren for about 6 weeks to make sure any in there cycle out
What was your feeding like, schedule and type? Variety seems to be key. I soak mine in Selcon and VitaChem, for added health. I feed veggie flakes twice a week (no tangs right now) and soak it in NutraMar Ova, to trick the carnivores that won't touch flake into doing so.
I lost every fish in my tank once, when my return pump electrocuted them and know how disheartening it can be. You'll figure this out and move forward again soon. As hard as it is now, you'll learn from it and the experience will benefit you throughout your tenure in the hobby