I was just given a tank that had two snails. Both snails have white spots at the center of the swirl in their shells. Is this a parasite or illness? Anything I should be concerned about? Thank you! To give some back story... These two snails are actually pretty incredible. The man that gave me this tank, drained the tank and moved the fish to a separate housing tank. All that he left in the empty tank was the brown gravel. He didn't realize there were still snails in the tank. The tank was empty for several days before I got it. When I filled it, one of my brown "pieces of gravel" started climbing the tank glass. Hardy little fellas.