Ironically, adding un-dechlorinated tap water to your tank (which only has no visible ill-effects on the fish, FWIW - it still strips off their protective slime layer and irritates their gills) is killing the beneficial bacteria in your tank that might help keep the algae under control. As well as maybe covering the tank with a towel or something when you don't need to be looking at it (to reduce the light that the algae uses to reproduce and grow - we call this a black out), also use a good water treatment like Prime and let your beneficial bacteria get a foot hold.