I doubt that extra potassium is of any value. Do you have a full dosage of phosphates? Any shortage in phosphates will stop the plant growth. Also, how do you know you have good CO2 level?
I think you have far too much light for the midday burst. 4.3 watts per gallon is really a lot of light, unless you can be sure everything else is being provided in concentrations good enough to meet the high demand that the light is driving. If this were my tank, I would only use two of those bulbs, and no midday burst.
Do you have very good water circulation, so all plants are swaying in the current? The lack of adequate circulation, with that much light, means the CO2 can be used up locally, with no new supply being brought in by flowing water.