I suppose you could, but why pay for water when tap water is fine for an aquarium. Heck, call your pet store and ask them what they think. I would imagine that you have to keep the water balanced with certain chemicals anyway and distilled water would intensify the effects of any thing you put in there. I would just use tap water. Cheaper !
Depends on the type of fish. If your fish needs a specific type of water, example: salt water. Then no, your water from the store would not be okay. Check with your pet supply carrier, they'll know what's best for your type of fish.