a 4" RES could easily take a pleco down, unless it was much larger than itself. common plecos dont eat algae anyways over a certain size, so all you would be adding is more waste, not something you want to do in a turtle tank. not to mention that tank isnt big enough if it is a female, and would be absolute minimum if it was a male, and even then you would have to have a ton of filtration, 100% weekly water changes, and the tank would have to be setup right to handle the large turtle. so yes, even if the pleco made it for now, it would end up eaten. so just scrub the algae during your weekly 50% water change (what i recommend at minimum with a water turtle or fish). if you want fish you can choose from fast hardy species. they will learn to avoid the turtle, and if you never feed live food to your turtle (as you shouldnt do anyways) the turtle will be much more tolerant of the fish. and if you put the fish in now while the turtle is smaller it will hopefully get used ot the fish and they will have better chance sof living. i have had convict cichlids, goldfish, and a few others with water turtles. in all likelihood you will still lose a couple fish, mainly because they dont learn fast enough to avoid the turtle. having fish also lets you make sure the water is staying clean enough. if the fish are doing well, you knwo your keeping the tank clean enough. what filtration do you have? what lighting? how deep is the water? what are the dimensions of the tank?
