At that light level you and if you are injecting CO2 at all, you may have to dose every nutrient that there is. Plants not growing will be the first signs of NO3, (nitrogen) deficiency. Curled leaves, clear spots, brown spots, leaves falling off, and many other things can be signs of deficiencies.
I only have 33 watts of NO flourescents over my 10 gallon tanks and I have to dose KNO3 (nitrogen), KH2PO4 (phosphate), K2SO4 (potassium), MgSO4 (magnesium), CaSO4 and CSM+B (iron and other micros). If I don't dose them all I get Staghorn, BBA, string algae and some really ugly looking plants in a short period of time. And I do not inject CO2. When I did my dosing was almost double what I do now to keep up with the uptake of the plants to even have a chance to control the BBA and Staghorn algae.
Take a look HERE
for more information about deficiencies and their signs.