While other turtles don't necessarily make good tank mates for turtles, some fish do. I have experience with two kinds of fish. Goldfish: I buy a dozen feeder fish and drop them into the tank or pond. The smartest and healthiest ones will survive and grow to create entertainment for the turtle and added beauty to your tank. You repeat this process whenever the number of fish drops below what you would like in your tank. My smartest goldfish have grown to several inches in size and have survived for over a year. Convict Cichlids: A colleague told me that he is raising Convict Cichlids in his indoor turtle tank. I was fascinated by the idea, so I moved the goldfish to the outdoor pond and acquired a pair of convicts. Within a few weeks they bred for the first time. The babies hatched, but then the turtle got the upper hand, because the spot the fish had chosen was not secure enough. My colleague has raised several generations of convicts in his turtle tank. While they are breeding, convicts are aggressive, and they can chase away a curious turtle. If you do wish to keep convicts or other cichlids in your turtle tank, make sure you provide ample hiding places for them. If your tank has a bare bottom, use some jar lids to provide areas where the fish can herd their babies. Other than the fun, there is another reason for keeping fish with your turtles. Fish are good indicators of water quality. As long as the fish are healthy, the water quality is most likely good. If the fish get sick and die, then the water quality is not OK.
Pond Mates
I keep turtles together with large and small koi, large and small goldfish, mosquitofish.
Look for more information in the pond articles.