Goldfish can be kept in an aquarium equipped with a filter, light, heater, aeration, plants and decorative castle. Check the water's ph level and add chemicals to keep it optimal. Figure one and a half inches of fish per gallon. Trouble is, fish grow. Like all rules of thumb, there is that pesky reality whereby reality thumbs its nose at you.
Aquarium husbandry has it's own challenges and advantages. For those who are looking for low tech, I give you the fishbowl.
It is possible to keep Goldfish in a bowl with no fancy equipment whatsoever. The fish above have lived in this bowl for several years (which is why the fellow on the right is so big!). You see, it can be done. Here's how:
Never remove/replace more than 1/3 of the water volume at one time.
Don't change the water too often.
Feed the fish sparingly.
Now and then dump all the water and the fish in a bucket. Wipe down the sides of the bowl with a paper towel. That greenish brown coating comes off easily. Rinse the bowl out a few times to remove the floating greenish brown stuff that used to coat the sides. By now, the water in the bucket has settled so that most of the fish poop has sunk to the bottom. Dump 2/3 of the bucket water back in the bowl along with the fish. (If you like, use that remaining water containing nutrient rich poop to water your houseplants).
Add some tap water that feels the same temperature as the bowl water but only about half of the 1/3 needed to fully fill the bowl. The next day add more water. This way the fish don't have to adjust to new strange water all at once. It is diluted sufficiently so they are stressed as little as possible.
Sure, living in captivity wasn't my first choice but at least I am safe from hungry Herons.