You say you have zero phosphates. You should have about 1 ppm of phosphate, not zero. I assume you are fertilizing the tank. If so, you should be adding nitrates, phosphates, potassium and trace elements, using a method that you can follow routinely, always dosing the same amounts. Many of us use Tom's Estimative Index method, which you can read about in one of the forums here. When you get the fertilizing going and keep the amounts you add about the same every time you add some, the plants should grow at a steady rate, which will discourage algae from starting to grow.

You also need to keep the same concentration of CO2 in the water during each time the lights are on, so you need some way to determine that you are doing that. Using a drop checker is the best way I know of to do that.