I have had a red-eared slider in my koi pond (approx. 1500 gallons) for the last eight years. Occasionally, he will swim after the fish and sort of try to nibble at them, but they are much to fast for him, and he has never caused any damage to them. Be sure there is some way for the turtle to come up out of the water and lie in the sun, which it will want to do periodically. In my case, there is a brick wall around the pond with about 18 inches of sunning area between the wall and the pond edge, and I use a log as a "ramp" for the turtle to get to it. A semi-submerged rock or log would do just as well. Incidentally, he seems quite happy eating the koi food (Hikari) that I use. Hope this helps.