Yeah, the old turtle in the fish tank question. It can work, but you always risk having some fish coming up missing. I keep softshells, so it tends to be a no-no unless the fish is a big bruiser.
And about water quality, turtles are about the same as fish in making waste. A turtle just doesn't produce more poop than a fish does. The thing that makes it seem this way is that turtles are often kept in tanks too small for them. Just like having a big fish in a little tank, it will get fowled often. Its like seeing a 10 lb snapper in a 75 gallon, now imagine a 10 lb p-bass (I'm not sure how big that really is but over 24" is a good guess) in a 75 gallon. Not a good sight.
And for the water quality affecting the fish in a turtle tank, fish are not the only ones to worry about. The turtle has to drink the water it is in so it effects the growth and health of the turtle aswell. And the water is what keeps the turtles clean. Clean water is as important to turtles as it is to fish.