There are several stores in my area that deal in marine creatures.
The shiny clean (nationwide chain, not petco) one does nothing for me. The staff seems intent on selling, not so much on good advice. When I do stop in there, I have on many occasions waited till the staff member left and then given the customer the correct advice. (I did retail marine for over 10 years, have been reef keeping for over 20) Most have thanked me. Their fish are often ick infested and over stressed. Their coral selection usually stinks. Prices not so great either.
Another store (another chain, not petco) is usually clean, but the staff disinterested, at best. Ick and dead critters often a problem. Prices sometimes very good. (they sometimes mislabel fish/corals)
A third place (corals only, maybe a couple fish, but not a money maker for them) seemed clean enough, and had a huge selection of corals and frags at good prices. They also had a staff member tell me that how I have my system set up will lead to a huge crash within 6 months, 1 year tops. Funny thing is, this is one of the tanks that has been set up since ~1990, using a 120g refugium with DSB & Cheato, and good sized pile of LR Rubble both in the fuge and in the 50g sump. Carbon and GFO reactors. Hasn't crashed yet!!!
The 4th place is the smallest, but only because it is crammed with tanks. There is barely room for dry goods, but what they have is top notch. They have an excellent variety in soft and hard corals, a good invert/cuc selection, and a surprising number of very nice and often unusual fish. They have frags starting at $5. More importantly, they have a staff who really seems to get it. The owner(s) are there pretty much all the time, and the employees can actually talk reef correctly! It is always my first choice - in fact, I keep trying to get them to move from the east side of town down here to me on the south end!