Black snails

they are good snails to have if they are breeding in your tank then you are doing some thing right they are Stomatella snails they come in many colors. They eat algae and that is about it from what I know about them. Here is some thing I cut and paisted from and site.

This saltwater snails body is half covered with a shell. When attacked it can drop its tail like a lizard. These snails are known to eat algae.

Bill