I"m no expert but here goes with some info
It all depends on which corals you would like to put in there. Usually the rule is 5-7 watts per gallon. This can be an expensive conversion...
Corals cost, lighting costs, many more additives and food cost, and you have to watch your nitrates like a hawk.
Corals are the real challenge that's why I'm lazy and I stick with fish only.
However, there is a way to dose your tank with a iv bag. You make a mixture and set it to drop in at a perfect rate. Then you have to make sure you do your water changes on time.. fish will accept the nitrates a little high but corals won't. Also, you have to make sure your fish are reef friendly or they will eat your expensive corals.
If you're ready for the challenge and the science.. then go get it! Results can be very rewarding.
Oops, I forgot you have a 20... Ok.. Some power compacts will do fine for anything. make sure you get about 110 watts. Clams require more light. mushrooms and polyps don't.