I've never checked for phosphates, I dont have a phosphate kit. I check for ammonia, nitrites, nitrates, and ph on a regular basis. I do use water right out of my tap, but I use NovAqua water conditioner and this is suppose to take care of all that "unwanted stuff" in the water. I do have a window at an angle about 25 feet from the tank that receives sunlight in the morning hours. Could this be whats causing the algae to bloom so quickly, the sunlight?