Steve, water changes are stressful also. You are raising tilapia and algae is not only a favorite food, it is also a substrate for beneficial bacteria. Algae needs light to grow and if the fish are covered, are you sure it is alage and not a bacterial, mold or mildew colony? When it comes to water changes, do complete ones not partial.
I would not add hydrogen peroxide...try sodium percarbonate "Green Clean", but I wouldn't kill the fish food (algae)
