They feed on algae and small creatures. They are grayish white to brown and black. Planarians are of interest to aquarium keepers because they can be introduced in plants and establish themselves in the sand. They are normally seen at night, where they are seen scouring the walls of the tank for food. They are harmless to adult fish but do pose a risk for eggs in a breeding tank.
Planarians can be quite difficult to eliminate.
Hanging a piece of fresh meat or liver in the corner of the tank can be successful. You can then remove the meat along with all the worms attached to it. There are parasite medications available to remove these pests as well. Snails - Many kinds of snails perform a useful function in the aquarium as they eat algae. Most snails feed on rotting vegetation, and leftover food.