Hi Woodyship,
Could you please provide some more details on your tank? Are you dosing macro/micro fertilizers and/or c02? Fixture/bulb type (MH, T5, CF, ??), filtration setup, powerheads, etc. Substrate type? Inert (gravel, flourite, etc) or fill of ferts like ADA? If injecting c02, how do you do it and how do you measure the c02 content in the water? Do you use a drop checker at all?
One to many/all of these factors may play a part.
500 wpg is WAY OVERKILL for your size tank IME/IMO. Light is what drives plant growth and thus nutrient and c02 demand. The higher the light, the higher the demand.
If the demands of some nutrient is limited (Leibigs law) plant growth will suffer.
The more info we have, the better an answer can be returned
In my head, I am thinking c02, but we will see based on more data/info.
Hope this helps.