The more light, the more the plants photosynthesize.
The more the plants photosynthesize the more demand they have for Carbon and Nutrients (nitrates, phosphates, potassium, Iron + trace elements)
Since algae is also stimulated, but NOT limited by the need for all those nutrients, herein lies the problem and the need for a balance.
Your substrate looks white.?
At 6.5 WPG youre going to want a hell of a lot of co2 and nutrients too.
Why is this?
a low tech with a lot of light without CO2 should work, right?