i do not disagree that tangs are beautiful center pieces, but in a 30 gallon tank??????
i really think that they need alot more room for swimming, and a 30 IMO isn't quite enough(for any tang), some have had some success with them in smaller tanks, but i have only ever heard of one or two
a sail fin tang is a 16 inch fish(not ot mention that tangs are hyper swimmers and need long tanks) i believe, aand a 30 gallon is how long???, NEVER BUY ON THE IDEA THAT YOUR FISH IS X INCHES, always figure their adult size when you buy for saltwater, sw fish do not grow to fit your tank, they grow period
besides, you only want one or two inches of fish per five gallons of water(one in a fish only tank without a strong biofilter), this leaves you with only a couple of smaller fish to keep, no large ones,
please please don't consider a big fish, for his sake, PLEASE!!!!