There are three main pods found in the aquarium, isopods, copepods and amphipods. The easy way I have found for differentiating them is as follows. Isopods crawl around on all legs similar to an ant and they do not have tails, copepods get around the same way but do have a tail similar to that of a shrimp or lobster. Amphipods look like freshwater shrimp and tend to scurry around on their sides. When they do stand upright they are hunched up. Pods are generally safe but there are species of isopods that are parasitic often attaching to the fishes tongue.