ive had a "chinese algea eater" in with my res for months now and it occasionaly gets a nipped fin thats it, the res is about 2inches and same with the alge eater. i also kept it with a koi of about 8inches with no problem, and 5inch commets with the occasionaly fin nip is all. and ive seen them in a pond at full size male/females with goldfish close to there size and no problems. it depends on the temperment of the personal turtle.