Assassin snails are a colorful counter measure for keeping unwanted snails under control. They usually hide in the substrate during the day and are more active at night. They do move fast, for a snail, to catch other snails. They will reproduce in your tank with enough snails to make sure you have females and males. They will eat other snails but will also eat sinking pellets or flake food. You can get the snails as a preventative even if you do not have unwanted snails.