I'd consider using this or another pendant (the coralife circular pendant is kind of nice, and the Geisemann pendants are beautiful but pricey) and going open top. Any mh bulb that close to the water on a such a small tank would probably cook things pretty well.

For comparison, I have a 50 gallon corner tank with alot of surface area open, my mh pendant is about 7" off the water, and it runs at least 6-7 degrees over room temp; that's with only about 60 watts of pumps, 45 watt external and 2 nano streams.