My wife got a goldfish from our pre-school. It came in a "fishbowl" the size of a large coffee cup and a tiny ziplock with fish food. It was severely stressed: fins clamped at his side, sunken belly, gulping bubbles at the top of the tank, flashing, drifting sideways, etc.
I had a used 3 gallon fish tank, but had no time to set up or cycle. So, I put the "fish-cup" in the center of the empty tank and over the period of days, added maybe 4-6 oz per hour of sink water to the fish-cup (I have a RO filter) until the tank water level exceeded the "fish-cup" and he could escape into the tank.
He did experience some shock (lethargy), but the flashing and side-drifting stopped. I kept adding RO water and removed and replaced the majority of the water almost daily.
Two days ago I started running the filter for about 4 hours during the day (on a timer). He's not happy about the water-current, but he is alive. He explores (I think he's looking for a way out) and hides in his cave when people are around. He's eating, resting, actively swimming. No flashing, no lethargy. His major stress comes from me exchanging water. He freaks out, but the water needs changing. Right?
I was going to start the kids with aquatic plants then eventually add ghost shrimp. Maybe a betta for Christmas. For all the prior neglect, this goldfish is in much better shape now. I know the tank is too small for the goldfish. I feed him twice daily. I exchange about a half-gallon of the water daily with a pinch of baking soda to offset acidity. But I am not going to do much more. I might add a plant, but I worry anything new might stress him. I've had him for a week now and he seems ok.
I've never had a goldfish before, so I don't know how they act. He seems to pace the tank like a caged tiger. Back and forth, up and down, front and back. After I add fish food, he waits... then stalks like a deep-water shark zipping up from the bottom and attacking at the surface. Is all of this "goldfish behavior" and him really needing a huge tank? Am I doing enough for his survival? If he lives a year, I might reconsider getting set up with a much larger tank. I just want to know if I'm giving him a sufficient minimum to survive.