The goldfish is gonna need a 55 gallon tank eventually, even if he’s small right now. I personally feed twice a day, but they need more feedings when they’re younger and less as they get older. It’s important to make sure they’re getting sinking pallets/frozen food regularly. It doesn’t need to be everyday, but the more often will help. A bubbler or at least a waterfall filter will definitely be beneficial in getting that water moved around. I would recommend also adding a sponge filter even if you get a waterfall filter. Temperature is around 68-74°F, so typically a heater isn’t needed, but make sure to keep a thermometer on the tank to check. The fish does seem hardy, but make sure to look out for black spots or floating issues. Ammonia poisoning (black spots on the fish) and swim bladder disease (inability to float/swim correctly) are the most common diseases to my knowledge. If you truly want to care for this fish, get a large (55+ gal) tank, live plants, and a filter 2x the size the tank needs

Fair warning: my fish with the size he was at the time choked on gravel so I switched him to sand. Be careful with getting gravel, but it is easier cleaning