The turtles will likely (but not always) pick at something to death. The pleco may avoid the turtle wrath for a long time, because turtles have poor eyesight and are slow. Eventually, the turtle will get quick and sly enough to catch what it wants.
Like rich mentioned, weekly large water changes are required for RES. I do about 80% a week on mine, any longer and it turns green in a heartbeat.
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