Looks like your kh and ph are very low. You might try adding a couple pounds of crushed coral to the substrate. Also your light is set kind of weak. Maybe bump it up to 30% at first, for a month or two to see if that helps. 40% might work well. Also, duration could be increased to six or eight hours per day. Your plants growing well at first then hitting the wall, so to speak, could indicate a need for more fertilizer.