My colleague has raised several generations of convicts in his turtle tank. While they are breeding, convicts are aggressive, and they can chase away a curious turtle. If you do wish to keep convicts or other cichlids in your turtle tank, make sure you provide ample hiding places for them. If your tank has a bare bottom, use some jar lids to provide areas where the fish can herd their babies. Other than the fun, there is another reason for keeping fish with your turtles.
Fish are good indicators of water quality.
As long as the fish are healthy, the water quality is most likely good.