You need to read the specific policy if you want to be sure.
Yes ... not so easy.
From my understanding of what is "common":
You are covered if your tank breaks and has a major flood, but not covered for slow leaks,
moisture issues, and other long term things. (Like with normal pipes)
You are covered for damage to house, but not to aquarium itself.
You are probably not covered by normal policy if an earthquake causes the flood.