Q: Okai, so this is WEIRD. I have a fish tank. It's rather large. A while ago we found a little shell type thing. We certainly never bought it. It looks prehistoric, kinda fossil-like. I was just looking at the fish tank, and I found at least ten of then on the gravel. Does anyone know what these are? Where they come from? How they got there? It's creepy. Also, I'm rather certain they're alive. Sometimes they cling to the glass of the fish tank. On the inside of course. It's weird.
A: You have a snail infestation. The live ones are stuck on the glass. The dead ones leave their shells on the floor. As the minerals are depleted from the shell, it turns white, like a bone. You can use chemicals to kill snails or you can buy a fish that eats them- might not be compatible with your other fish, though. Or you can pick them out by hand using a powerful aquarium vaccuum. Snails are generally harmless nuisance, but might eat plants, if you have them. Snails also eat algae.