You need to find out the source of the algae problem before purchasing more stick to fix it. How long has your tank been set up? How long do you leave the lights on? If you do not have live plants, start by leaving your lights off all the time except when feeding or viewing the tank.
I would never suggest a common plecostomus (unless the tank is at least 200 gallons) because they grow to two feet long and have an enormous bioload...