A single 175w will do great. you will want to get a more sophisticated reflector (search lumenarc, lumenbright and lumenmax) to get full coverage. these reflectors allow you to raise them higher which will give you better coverage and heat reduction but still give you great intensity compared to typical batwing or lesser developed reflectors. Fan cooling should be sufficient for a single 175w halide over a 20g
You will be able to keep anything with a single 175w. Leathers, xenia, star polyps and zoanthid LOVE high light, more so than most LPS IMO. because they dont require it theres a misconception high light wouldn't be optimal for them. the REAL problem with leathers, xenia and star polyps under high light is they grow TOO FAST and too large too quickly. In a small tank like a 20g if you cant get rid of it fast enoug or allow it to grow beyond frag plugs they will begin to take up too much real estate or become nuisance. I think (well I know since I've used a high par 175w bulb) 175w are a good medium for not growing fast growing corals too quickly yet strong enough to keep the high light corals. If you want a frag tank with SPS dont do leather frags. if you dont want to do sps you can do whatever. Its a pretty small tank and leathers are known to intiate chemical warfare. In a frag tank you'd probably have several and there's not going to be alot of dilution (heavy carbon use may be effective against it).
