i have a 150 20k hqi MH coralife pendant on my 20L and i had to raise it over a foot off the tank to make life good for the tank. if the tank is 18" or so, you really shouldnt have a problem. it should be ideal. you may have to toy with the hight of the light above the water. if you are concerned about too much light, i have heard of 70w MH lgihts for aquariums. i think hellolights.com has them.