the coverage is at least 36"x 18" from my experience. I had it previously over a 40gal breeder and had a lot of coverage. I still get coverage for a 54"x18" tank, but the intensity of the light is cut because the fixture is hung higher. My current tank is about 100gal with the dimensions of 54"x18". Bulb wise, I use a more 14-20k coloration of blue pluses, actinics, coral plus and purple plus. However, the bulbs are past expiration and need to be replaced (which is a given when you buy a used T5 fixture). Once you pop in new bulbs, there's a huge difference.
