You simply have waaay too much light. I was using that much light with pressurized co2 and a full fert routine. You have two options:
1. Start inject co2. Excel isn't gonna cut it.
or
2. Reduce your lighting, by 50-75%. Raise the fixture about 2ft off the tank, or get a new fixture that uses T8/T12, or PC bulbs. T5HO are too intense for non-co2 tank.