If his tank is getting a lot of light, trying to reduce the amount that the tank is exposed to might help along with the plecos. Try a piece of black cardboard or poster board that you can use to block out the light for the times during the day when the light is strongest. Since you can't move the tank, this may help along with the plecos and increasing the water flow.
Also make sure you aren't overfeeding and introducing excessive nutrients into the system.