Have these lights! My tank was 21 inches deep. Lights mounted directly to rim, no risers. Apogee Par meter put the two at 50-80 par DIRECTLY below the lights at substrate and around 200 par right under surface of the water DIRECTLY under the lights. And I mean that meter was right below them. However moving off center even an inch had a sharp drop off of 40+ par which got worse the father from the lights. I found these lights, even running two, to be too weak for high demand stems and Co2 at my depth of 25 inch deep tank 21 inches from water surface to substrate. It did not have a linear drop at all, very unforgiving, off center or closer to substrate had very sharp drops. IMO humble opinion the lights are more suited for shallow tanks. My profile pic is the Petco Basic with the two fluvals. Notice all low light plants, tank was low tech. It worked well there.