When you do water changes, you remove all of your fish? This is basically unnecessary, just leave at least 25% of the water in the tank and your fish will be able to swim just fine. Always remember to use a water conditioner such as Prime which is made by Seachem. It is a great product and recommended by a vast majority of forum members here. Your weekly water change should be between 25% and 50%.
What are your exact water parameters? Ammonia, Nitrite, Nitrate? and others if you have them, they may be very important to your problem.
Also, assuming your tank is cycled, and you perform proper weekly maintenance you should not end up with much algae and shouldnt really need a pleco to beginwith. If you feel you need/want a pleco make sure to do a 2 hour drip acclimation for your tank next time, it sounds like you might need it.