I believe that tank is 16 inches high and 24 inches long. With that height and that light you definitely do have high light. But, with high light you have to use CO2 and you have to dose nitrogen, phosphorous, potassium and a complete trace element mix. You aren't doing those things, so the plants will be unhealthy. You can raise that light 6-8 inches above the top of the tank, reducing the intensity down to low to medium. That will reduce the plants needs for all of those nutrients, and they will be much healthier.