What type of turtle is it? sliders or cooters will kill the plec in that, small enough for him to catch it no matter what size the plec. is, tank. a small map or painted turtle will do fine though. in order to keep turtles with fish you will need a lot( think at least 300 gal.) of space for the fish to be able to distance itself from the turles or else any omnivorous or carnivorous turtle will kill any fish even close enough in size, meaning less then 7 inches bigger then the turtles. I tried it before. unless you have slow turtles like mud or musk which will still kill a sleeping/resting or slow fish.
A chocolate cichlid gets to 10 - 12" & thick & pretty tall. 20 gal is too small.