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.
The snail also has an appetite for fish spawn. You should remove them from tanks in which you are using for breeding fish.