Tank Mates for Your Turtle
Many often wonder what other living creatures one can add to their turtle's tank. There are many possibilities that one could explore, however consider the high likelihood that the turtle may eat what is put in their tank. Because of this I would start out cheap and not buy something expensive. Also when introducing the tank mate to the environment it may help keep it safe to feed your turtle before or during the new items release. If your turtle is hungry it is more likely to attack the new member of the tank.
What Not to Put in Your Tank
When thinking of tank mates many think of other animals that they would like to put with them because they think they have similar habitats in the wild or they will look cool together. When thinking of a tank mate you can take aesthetics into account but you must also realize what the tank mate's requirements are and if they could be hazardous to your turtle or your turtle hazardous to them. Many Amphibians are not a wise choice unless you are very experienced as many of them excrete toxic chemicals from their skin and have soft bodies which can be harmed by turtles. Other Reptiles are not the best decision either as turtles produce a lot of waste and could present health issues for other Reptiles. Just because animals can live in the same environment in nature doesn't mean it is wise to put them together in captivity.
You can put crustaceans such as shrimp in your tank and many can be beneficial to your tank by eating some algae and food scraps off of the bottom. Many turtles eat shrimp, crabs, and crayfish so it may not be wise to put some of these in a tank with a turtle. I have found that ghost shrimp have a decent survivability rate if you give them appropriate hiding places and cover. I would not expect to have a thriving ghost shrimp population in your tank however as turtles will eat some of them and some may die on their own as they do not have terribly long life spans. Other crustaceans do have the possibility of harming your turtle as many have powerful claws but they will still more than likely end up as prey for your turtle.
Snails can be a great addition to your tank as many of them are omnivores and eat leftovers from your turtle. There are many varieties and, like anything else you are going to put into your tank, it needs to be researched. Mystery Snails/Apple Snails are a good choice and will help clean your tank. They do eat a little bit of algae but it is not expected that they will rid your tank of algae by any means. Depending on the size of the snail it could become a meal for your turtle. It is just a gamble if it will be noticed or not.
Fish can be a good choice for a tank mate for your turtle. When choosing a fish you would like to keep with your turtle make sure you research the fish thoroughly. The fish you would like to keep may not be compatable with your turtle due to specific water conditions or other reasons. I find it to be hit or miss when picking out fish that won't be eaten but I would suggest that you pick something that is fast and possibly a little aggressive. The best way to keep your fish alive is to make sure that your tank is not bare and that there are hiding spots for your fish. If you have an barren tank it makes it easier for your turtle to chase the fish down. If you have some plants and a hollow log or something of this nature it breaks up the tank a little. By doing so it makes it harder for the turtle to track the fish down. Many want to keep plecos and other fish that eat leftovers and algae but this may not end so well for them as they may get some bites taken out of him/her, as it doesn't move much. Research is imperative as you cannot rely on the staff at a chain pet store to be able to give you correct information on the fish you are thinking about getting. Some like to keep feeder fish in their tank so their turtle can get a snack sometimes. Eventually the ones that do not get eaten grow bigger and become tank mates. When picking out feeder fish there are a few choices but you should try to stay away from goldfish as they are fatty and suposibley not that good for your turtle. Instead try using rosy red minnows or some other type of minnow as these are a more nutritious choice.
Gold Fish and Koi
If you are wanting to keep goldfish with your turtle as a tank mate and not as potential food I would go with a comet goldfish, as they are quicker, or even a shubunkin goldfish would more than likely do alright. As for Koi they are not a bad choice but they can get rather large and would be a good choice for an outdoor turtle tub or turtle pond. These fish are nice to have in your setup because they are omnivores and eat leftovers from your turtle, which is nice because turtles are messy eaters. You do need to watch how many of these fish you put together with your turtle as these fish create a lot of waste and you do not want to overload the tank with ammonia.
Some choose to keep tropical fish, such as Cichlids, with their turtles. Some tropical fish get along well with turtles but you need to be sure that their water requirements are in your turtles requirements range. I have some Cichlids with my turtles and they seem to be doing great. Some Cichlids are more aggressive than others and may kill other fish that you have in the tank with them. Also some Cichlids get pretty big and may bully or out compete your turtle for food. Cichlids that I have are smaller varieties from Africa and all are mildly aggressive except for one aggressive Auratus. With these fish particularly I would do a lot of research before getting some. I went to a chain pet store and asked the aquatics staff member about the differences between the different Cichlids and which was the most aggressive. She failed to tell me that the Auratus was very aggressive. I bought two of them and put them in my tank with my Red Ear Slider. I thought everything would be ok but the two Auratuses ended up fighting and one of them killed the other. I had not provided ample hiding spots for the fish and they ended up fighting over territory. Despite these fish being aggressive, you can keep un-aggressive fish with them as long as they are a good size larger than the aggressive fish. The aggressive fish may harass the larger fish a bit but it should be ok until the aggressive fish gets closer to its size. Cichlids can be a great choice for your turtle tank as they are fast and I believe not likely to be eaten if given proper hiding spaces. Not to mention they are some of the most colorful fish in the world.