It sure seems like your plants are starved for something. You have plenty of light, a good substrate, and Excel works as a carbon source, so if I were you I would try dosing KNO3 and KH2PO4 for a month, just to see what happens. With only Excel for carbon the plants won't grow at top speed, so you won't need the full EI dosage of any fert, but dosing half that amount should work, if that is the problem.