I wouldn't use tap water, unless you test for everything and it all works out. I learnt the hard way, when we moved to the acreage I didn't even consider that the tap water might be unuseable. Turned out it was high in KH
and im sure alot of other levels were way off too.
You could also try a coral that doesnt need light, like the suncoral. Ricordeas would do pretty good probably as well, the orange ones always really pop.
*edit* actually suncoral might not be the best for such a small tank, they require to be target fed which would dirty the tank up pretty quickly. Could try feeding mysis shrimp instead of a liquid additive though probably. Anyone else know if that would be bad??