Only use RO/DI everytime you do a water change and top off, tap water is just adding to your problem, get a TDS meter and test your spring water/tap water, you want 0 TDS or your just adding problems from the start. Increase water movement that will help out greatly for the dead spots where cyno is forming. Also what is your Alk, Ca, and Mg levels. Get them in check and keep them there. Also if you are reading 0 phosphates there are different types or phosphates some that can not be read by test kits. At the same time as soon as phosphates are present they are being absorbed by your algae. RO/DI will save the headache and work on the algae battle, save up some $$