I'm using easy green ferts..I now am running co2 and higher lights. How often should I use easy green with a high plant load..low fish load on my 40 gallon tank?

One way to tell how much ferts you need is to dose as frequently as possible, once daily, the idea behind this is you can dose less if you get algae . Vs not dosing enough and having plants die back. But that depends on what plants you have.this is based on the Tom Barr EI method. I believe Aquarium Coop has a video on this , where he states it's better to dose more frequently than just dose more . Try searching for " Aquarium Coop fertilizers " on YouTube. Hope this helps ! Have fun!!

4 pumps twice a week is the recommendation, could be more or less depending on Bio load, plant species and how long the lights are on.