IMO The fish Store is the best place in town to buy rare fish, monster fish, great looking, healthy fish that are fully grown or lovely large live plants for a very low prices. Also, great selection of chiclids
PetsMart on North 27th has a the best selection of plants with a fairly knowledgeable crew. When I bought my Baby O they were very quick to make sure that he would be properly housed. They do not have much of a selection when it comes to hard to find fish but if you are looking for something common this is a good place to go as the prices are lower than The Fish Store.
I have only been to Fish World once. They had a nice selection of beautiful full grown discus, healthy looking albino salamanders, and fairly pretty Koi there but the owner seemed know nothing of plants.
PetCo on O street is probably my least favorite place to purchase fish as the staff is fairly undereducated on both plants and fish. They is a large redheaded man who is most unpleasant...I believe he is the manager of the department. I stopped going when I decided to ask about the obviously undernourished Speckled Bichir that they were selling there housed with a bunch of chiclids. I asked about the poor little guy and they told me that the have had him for three months and feed the tanks flake exclusively....meaning that the fish was starving because they did not think about the fact that the speedy chiclids would eat up those flakes before they hit the substrate and even if they did not...flake is not suitable for the fish.