Well unless the fish put out a lot of waste (ie: goldfish put out a lot of ammonia etc) then you shouldn't need anything more powerful. For aquatic turtles its suggested you use a filter 2 - 3x powerful than the size of the tank because they put out more waste. But you should be fine with one for just a 10 gallon tank. If you WANTED to get one for a 20 gallon tank you could, it wouldn't HURT