I’m no fish expert but I’ve owned a number of goldfish over the years. Usually just one or two at a time, however. Here’s what I operate by:
Goldfish don’t need a formal “tank” with aeration, filtration, etc. They can do just fine in a goldfish bowl (wonder where that name came from ) or other receptacle - even a vase. A smallish goldfish can easily live in a one gallon bowl.
Put those small stones (made for aquariums) in the bottom. I’ve always wanted to use those round glass beads used for flower arranging, but they’re more expensive, so I’ve stuck with the gravel.
A plant can make it nice, but also isn’t necessary. I do try to put a rock or big shell or something in the bowl to make it more interesting for them and to swim around/hang out near.
I’ve always used flake food - the goldfish I’ve had haven’t taken to the pellets too well. Get goldfish food, not betta or tropical fish food. Don’t over feed. Feed twice a day. Feed “only as much as they can eat in about 5 minutes” the food can always says.
Don’t clean the bowl too often - really. When it starts getting green is fine. You can change about 1/3 of the water in the bowl every week or so (see more on this below) rather than have to change the whole thing.
The biggest thing to NOT do is shock them with rapid water temperature changes. Always let the “new” water sit in a separate container for a while near the tank/bowl to equalize the temp with the existing water. If you don’t add those dechlorinization/demetalization drops, let the “new” water sit at least overnight to let the chlorine evaporate or whatever it does.
Don’t ever use soap or glass cleaner on the tank/bowl. No matter how much you rinse, you probably won’t get it all - bad for the fish! Usually I rub the pebbles on the sides of the bowl when cleaning/changing the water to abrade away the gunk and this works fine.
So, when it’s time to change the water, I usually let a large pot of water sit out overnight next to the fishbowl (the “new” water). Then I pour the “old” water out of the bowl and down the sink - pouring or having scooped the fish into some small container for the duration of the process - a glass or bowl works just fine. I’ll rinse the bowl and the gravel multiple times with tap water - until it runs clear - using the gravel as a cleaning agent as described above. Last rinse is with water approximately room temperature. Pour out all the tap water, pour in a little “new” water and swish around then pour it out too (to “rinse out” all the tap water). Fill tank/bowl with “new” water. Add fish back in. DONE! I did this just this morning for my 2 fish and it took about 10 minutes total (set the “new” water out the day before).
I’m sure someone will be along to tell me what I’m doing wrong. But my goldfish seem to live longer than anyone’s, so it can’t be that bad.
Have fun and congratulations with your new pet!