That Petco (Union Square) has very few experienced people working there. The fish are usually very cramped (5 tangs and 5 triggers in a 20 gallon tank is common) and don't look great most of the time.
To give some examples of things I've personally seen there...
I noticed how the acclimate their fish.... and it wasn't very profesional... example: Opening up about 20 firefish (packaged individually), dumping each one into a net, which was out of the water and each fish landed on top of another, then dumping the net full of firefish into a tank... THEN pulling out the floaters.
I've also seen them sell an obsene amount of fish to someone who really had no clue about saltwater tanks and thought they could just put as many in as they wanted all at one time. (The guy had 6 fish and 3 anmones for a 20 gallon tank) He was also asking why the 4 other fish he had weren't eating and acted wierd in his tank... he was told "I can't tell without seeing the fish".. that was the end of the discussion.
With that being said... I have to admit I've purchased there a few times over the past 3 years with mixed result... my suggestion is to ask when they are getting fish in and go at that exact time if possible... It's usually Thursday late afternoon....
The last fish I bought (6 line wrasse) I stood there for 1 hour as they unloaded their boxes from overseas... I then asked for a 6-line and picked one out straight out of the coolers they were transported in... it never went in their tanks... so far (2 months) he's been great.
The only benefit... if there is one... sometimes you can get a misidentified fish cheap there (lack of experience on their part)... When I bought the wrasse, they had a
Red Sea Sailfin Tang (usually 80-90 bucks) labeled as a "Common" Sailfin (only charging 33 bucks).
So I'd say buyer beware... but you'd probably be better off at New World, or in some of teh other places like Fishtown or Absolutley Fish.
J.