What is this shell?

[paul monfils]

I can't say anything for sure but I suspect that the "seashells" you refer to might simply be aquarium snails that were not removed by the previous owner, and simply allowed to die and dry out. When you added water the dead snails may have started to rot, causing bacterial and probably also fungal contamination of your tank. This can happen if a fish or other animal dies in your tank in a place where you don't notice it. It can also happen if you put more food in the tank than the fish and other inhabitants can eat.