What you are experiencing is very common, and it doesn't have to be. Fatal parasites like velvet, ich, and brooklynella only effect fish, not coral or other inverts. Tangs are very prone to ich and other parasites. This will just happen again if you don't set up a quarantine tank and use it with all new fish. Its very common for fish to look just fine, but still carry deadly parasites. Your tank still has forms of whatever parasite killed your fish and no fish should be added for 9+ weeks. I'd do some serious reading before you buy anything new; knowledge of fish diseases and parasites is basic and vital. You cannot rely solely on the advice of any one person; especially a dealer. I'd read the stickies at the top of the new hobbyist and disease section of the forum; also read a good book on the hobby. I suggest "The Conscience marine Aquarist" (B. Fenner) or similar book. Not to rub salt in the wound; but any decent source would tell you that a 55 gal. tank is too small for tangs. Always do some research on new fish before buying.