If you have the dimensions of 12x24 than that is an Eclipse II hood. I believe that the only retro-fit lighting for an Eclipse is 32 watts PC. That means you change the existing hood and the bulbs to allow for more lighting, and would allow you to keep mushrooms. As I previously said, you do not need to upgrade lighting to keep a marine tank, only a REEF tank. There are plenty of organisms that can be kept with an Eclipse system, but be sure that you add on a Protien Skimmer or additional filtration such as a cansiter filter, and do weekly water changes.
If you decide to get rid of the Eclipse hood, there are several PC lights that you can get to stand alone on the tank. I put a polycarbonate custom fitted hood so that the evaporation rate would not be too high and my fish wouln't jump out.
