So, you have two rows of bulbs across the top of the tank, and the two rows are roughly 15 inches apart? That means each spot on the substrate is getting light from 1 bulb, about 21 inches above that spot. If those bulbs each have an AH Supply quality reflector, you probably have 35-50 PAR, good low light to medium light. But, if the reflectors are not nearly that good (white painted flat surface behind the bulbs) you only have about 15-20 PAR or very low light. And, if the bulbs are in a tall canopy, say 6 inches above the top of the tank, you could have way too little light.