Aside from large predatory fish or crocodilians, few animals represent a danger to captive turtles -- but hungry turtles may dine on virtually any creature they can catch and overpower. Accordingly, it is wise to select animals elusive enough to evade the turtles or equipped with enough armor to avoid becoming dinner.
Your turtles are more likely to live peacefully with other animals if they are well-fed. However, use care at feeding time, as the excitement accompanying the event may cause turtles to inflict inadvertent injuries on the other critters living in the tank.
While most aquatic turtle species will eagerly consume fish when possible, especially quick or large fish are not likely to become food for your pets. Guppies, for example, are often too quick for turtles to capture, as are neon tetras (Paracheirodon innesi) and rosy barbs (Pethia conchonius). If you would rather keep large species, you can opt for algae eaters (Plecostomus spp.), sunfish or large catfish.
Large aquatic snails and clams are often capable of surviving alongside turtles, thanks to their protective shells. Many snail species will help to eat the algae growing on tank surfaces, thereby providing an additional reason to include them in your tank. Turtles -- especially mollusk-eating species, such as map turtles (Graptemys spp.) -- may dine on small snails, so try to acquire the largest specimens possible.
Many turtle species will eagerly consume crayfish. However, smaller species -- such as musk turtles (Sternotherus odoratus) or mud turtles (Kinosternon spp.) -- are unlikely to attempt to consume large crayfish. However, if you provide crayfish with ample hiding opportunities, they may be able to survive alongside painted (Chrysemys picta) or slider turtles (Trachemys scripta). Before introducing high-cost crayfish to your aquarium, start with low-cost species to learn how your turtle will react to the edible invertebrates.
Often, the best animals to house with turtles are other turtle species. Most basking turtles, including painted turtles, slider turtles, map turtles and cooters, will cohabit peacefully. Mud and musk turtles usually cohabit well with each other, although you should avoid keeping more than one male per tank. Large turtles may hunt and consume smaller turtles, so try to ensure that all turtles in a given tank are similar in size. Avoid mixing aggressive species, such as common snapping turtles (Chelydra serpentina), alligator snapping turtles (Macrochelys temminckii) or softshell turtles (Apalone spp.) with other species.