Summarising the last few posts:
Size of tank - volume and dimensions
How long it's been set up
Ammonia, nitrite and nitrate levels
Hardness of water
Temperature of tank water
Fish in the tank - both still living and those which have died.
For example, it could be the fish which died needed a different hardness. Or some fish needed a different temperature from others. Or some fish just don't get on with others.