You mentioned the snails dying first. What killed them and what are your nitrate levels (you don't mention them). In my first tank I struggled with nitrate until I got a HOB
refugium/skimmer combo. When my nitrate levels got quite high my turbo snail died. That caused more problems but fortunately no further deaths.
If you do have high nitrate that may have killed the snails and helped in killing the fish. Maybe.