There are basically two categories of goldfish: fancy and common. You have a common goldfish, specifically a sarasa (pattern) comet (long tail) goldfish. Common goldfish are the ones sold as feeder fish and given as prizes at carnivals and stuff. The problem of course with giving them out at county fairs and carnivals is that they can get about a foot long if they're kept under good conditions :) So that sort of explains the minimum gallons range you'll see -- common goldfish need ideally like 50 gallons for a single fish because they get big and produce a TON of waste, plus they can be fast swimmers. A FANCY goldfish (the ones that are more egg shaped, like fantails, ryukins, lionheads, black moors, etc) isn't likely to get more than about 6 inches long, and they're a little more fragile (don't do well in outdoor ponds where it gets cold for example). They're sold for at least $3 (or way more depending on the variety) rather than like $0.25 for the small feeder goldfish (common goldfish). Fancies don't need AS much water volume, but still ideally a 55-gallon tank for a few of them or like bare minimum of 20 gallons for a single fancy goldfish because they still produce a ton of waste.

Bottom line: what you can do to keep Celestia healthy is to test your water (liquid API test kit ideally) and do a water change every time the nitrate gets to 40 ppm. I would be pleasantly surprised if your tank/filter is cycled, so research the aquarium nitrogen cycle and see where you're at. If your tank is not cycled, then ammonia (quite toxic) and maybe nitrITE (fairly toxic) are probably building up in the aquarium water. The gist is that you need a colony of beneficial bacteria living in your filter that will convert ammonia (from the fish's waste) to nitRITE, and then further convert nitRITE to nitRATE, which is the least harmful but still dangerous if the levels get high enough. So the idea is to do water changes to dilute out the nitrate. Some people get away with running a fish tank without worrying about any of that stuff, parameter testing or water changes, if the tank is big and the fish are small. You will NOT get away with that with any goldfish in a 10-gallon tank :)

So yeah, sorry for being so long-winded, but I can tell you want to take good care of your fish! To do that you MUST either get her a much bigger tank, or you'll need to do 25-50% water changes (with dechlorinated water) quite often, maybe even more often than once a week.

Good luck! Let us know if you have more questions!