Copepods and isopods are little critters that live in and around the rocks and sand bed. Fish eat them, I think that they eat fish poo after it breaks down a bit for lack of a better way of putting it. Good critters to have plenty of.
Not all isopods are parasitic. The majority of isopods in captive systems are Munnid isopods, which are herbivores, and not a problem at all, and are great food for fish.