on the right track there buddy that second light should be able to do most coral as long as you keep the higher light demanding stuff up in rocks. the General rule is one bulb(row) per 3 inches depth, you have a 18 inch deep tank I believe so a 6 bulb/row would be recommended. the aquatic life fixture says 8 bulb which is misleading its actual 8x 36inch bulbs so 4 rows