Well seems how T5HO's dont come in any other size smaller than 24", your only other practical choice would be a metal halide pendant. If you went with PC lamps, then a squamie or deresa would work, but these EASILY outgrow a nano. HOWEVER, if you have never had a reef tank before, dont get a clam... simple as that. Give youself a year to read, read, and read, observe, learn and read! Another thing, it is difficult to keep a stable temp, pH, alk, Ca, etc that a clam needs to properly thrive. Lighting is only one of the aspects to consider. Eric Bornemann is an excellent author as is Anthony Calfo, Ron Shimek, Steven Pro, etc.
Hope that helps and :welcome to nanotank.com!
Jesse