A good fish store will be a great asset to your experience in the fish-keeping hobby, while a bad one can frustrate you enough that you'll want to give up. How can you tell the difference? Do a bit of background reading (like this article) and check out a few stores before choosing one. When visiting a store:

Look at the tanks.

The aquariums should be clean, with clear water and no muddy debris down in the gravel. The only exception to this is tanks with natural driftwood, which tends to "shed" and may stain the water a yellowish color (it should, however, still be clear). A bit of algae is to be expected, but uneaten food, fish waste, or cloudy water should raise an eyebrow.

Look at the fish.

Take a few moments to watch any fish that you might be interested in. They should be clear-eyed, nicely colored, and behave appropriately for their species (a pleco that swims erratically around the tank or a zebra danio that lays on the bottom are both bad signs). If you see any fish that display signs of illness, such as poor color, clamped or shredded fins, sluggishness, cloudy eyes, white patches or lesions on the body, or signs of ich (white-spot disease), pass on any fish in that tank as they may all be infected and it's not worth the risk.

Talk to the staff.

If possible, visit the store at a time when they're not busy and chat with the employees. Ask some questions that you already know the answers to and see how they respond. Sometimes there are several "right" answers, so listen to their ideas and see if they make sense (this is where having some basic background knowledge will come in handy). Educated staff can be very helpful when it comes to identifying and correcting the many problems that you may run into, from a filter that's not working properly to treating a disease. Most people work at a fish store because they love fish, so they're usually happy to talk about them, provided that they have the time and you are being polite. If you become friendly with the staff, they'll often go out of their way to help you out, perhaps by calling you when a fish you've been waiting for finally arrives or reminding the owner to keep the filter cartridges you use in stock.

Don't forget local, independent shops!

In my experience, these tend to be the best. They often have the most educated and helpful staff, the best selection, and the healthiest fish. For instance, both independent stores I worked for ordered their angelfish from local breeders because the fish arrived much healthier (angels tend to ship poorly) but the chain store I worked at shipped theirs in from Florida and they always dropped like flies. Besides getting a better product, patronizing local stores keeps the money in your area and, with the economy like it is, your money may even help keep the shop open! Check your yellow pages listings under both pet store and aquarium to find some of the smaller places that can't waste money on advertising.