Are you sure the tank is cycled? Letting the tank run for a week and then getting fish won't cycle it. Do you have a test kit to test the water? I'd do a very large water change right away (use good dechlorinator, Prime is best if you can get it). Then get a good test kit (API Master is best) and test the water every day; any time ammonia or/and nitrite are over .25 do a water change to get them down as close to 0 as you can. Same with nitrates over 20. Unfortuntely with a goldfish (which are extremely messy) in a 1 gallon you're going to be doing a lot of water changes. If you can return it until you can get a bigger tank, I would do so. Until then, keep up with the water changes. Even then I think it's a losing battle with a goldfish in a tank that small.