Test your water daily.
Your tank is Not cycled. You are now doing what we call a Fish In - Cycle. Your Ammonia can climb to dangerous levels quickly. If you don't have one try to buy an API master water testing kit. I believe they run around $22 on Amazon.
Typically I find it's better to have fancies with other fancies as the common goldfish outgrow and outcompete them for food.
For now do not feed any floating food. It is dangerous for the fancy. If the food you have is flake or floating pellets presoak until they do not float anymore and then you can feed them safely.
1x Weekly you can feed frozen, thawed, skinned, green peas (halves). Squish them a little bit if the fish are small.
Goldfish also like fresh , rinsed organic greens such as zucchini and pieces of Romaine. You can use a veggie clip or weight down the piece with a rock. You can blanch them or just let them stay in the tank and soften.
With GF in a new tank I'd get a siphon as you'll be doing a lot of water changes.
Clean water is the best preventative [emoji1360][emoji1360]
Spitting and chewing food is normal. Stringy white poop can mean some things, but go slow throwing in meds.
Try peas for constipation first.
SolidGold on YouTube has excellent GF advice. She breeds fancies.
Enjoy your tank. You will need to upgrade as your GF will outgrow a 20g.
But for now it's fine [emoji2]
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