I think that your tank is too small for a Yellow Tang. If you do want a Tang, check out the Ctenochaetus genus, they can be kept in a minimum of 4' of tank. That genus includes the Two-spot Bristletooth Tang, the Tomini Tang and the Yellow-Eye Kole Tang to name a few. As for the Box-fish, I do not think they are reef-safe if you are thinking of going with corals or inverts.
Tap water isn't a problem right now, do you have city or well water? I would personally switch to RO/Di water with water changes and top-offs, but that's me.
Last edited by wake49; 04-13-2010 at 11:42 AM..