How often are you changing the water ?
I would look at changing it (completely) about once a week.
If you can find a home for the pleco, you can easily remove the turtle every week for about an hour, and scrub the tub clean, then put everything back together and return the turtle.
Also - what kind of lighting are you using ?
Aquatic turtles generally do best with a full spectrum (UVB) flourescent reptile bulb, as well as a 'heat' light over their basking area.
http://www.wetwebmedia.com/FWSubWebI...CareBarton.htm