In nature, turtles coexist with various other animals as well as with other turtles. In captivity, tanks and enclosures can lead to overcrowding and territorial disputes. Your turtle is most likely content as a solitary pet. After spending the money to get your aquarium or terrarium set up just right, you may want to add some other animals, but it's important to consider the health and well-being of the turtle you already have before you do.
Basking turtles, such as sliders, can be mixed together; however, turtles can be territorially aggressive, especially during courtship and mating. It's important to be sure that you mix only males or only females unless you're prepared to be the proud owner of up to 30 baby turtles. Mud and musk turtles can live together, but if they get aggressive they need to be separated. A basking turtle and a mud or musk turtle is more successful combination. Snapping turtles and softshell turtles should be kept alone.
Young turtles may get along in the same enclosure, but it's important to keep an eye on their behaviour as they grow into adults. Unless you are mixing basking turtles and mud or musk turtles, make sure both turtles are similar in size. Make sure there are enough hides (boxes or shelters just big enough for the turtle to fit in) for both turtles, and make sure both turtles get enough food.
The common plecostomus is a type of catfish that eats algae and can grow up to 18 inches in length. It is commonly kept with turtles without problems. Other types of plecostomus, such as zebra and bristlenose, may not eat as much algae and may be eaten by your turtle. If you choose to get a plecostomus for your enclosure, choose one that is bigger than your turtle. Keep an eye on the fish and the turtle; though unlikely, each can potentially harm the other.
Chinese Algae Eater
The Chinese algae eater is a quick little fish that evades turtles well. As its name suggests, it also eats algae, though more so when it's young. These fish may survive with your turtle, but as with all fish, don't get too attached right away. This nervous little fish will probably manage to stay away from your hungry turtle, but turtles do like to eat fish.
If you're not set on having a permanent companion for your turtle, you can always get feeder fish and enjoy the variety in your tank until the turtle catches his snack. Ghost shrimp are cheap and help clean your turtle's enclosure. They also give turtles a chance to hunt. Snails are another feeder option. Mud and musk turtles especially like snails. Like ghost shrimp, they help clean the tank, and they may survive until they are too large for the turtles to eat.