Keep in mind that red-eared sliders will prey on smaller animals so they will try to eat most fish with them in the tank. Bigger fish like koi or plecos may endure them but they may still get their fins bitten from time to time. Get smaller, easy to replace, guppies or algae eaters that are fast. Goldfish and minnows are also good choices since they are cheaper to replace. If you plan to keep fish in your turtle tank, make sure you have a big tank with lots of areas for your fish to hide in like rocks or plants. Also, make sure that your turtle is well fed so it doesn’t try to snack on your fish.