Is it okay to keep a goldfish in a bowl?
If you have one or two fish, a goldfish or guppies, a bowl may suffice. The fish should be small (no bigger than one which could fit on a tablespoon or two that could fit on a teaspoon in at least three quarts of water). If you go the bowl, you need to do a 100% water replacement at least once a week to refresh the oxygen level. This is easiest done by scooping the fish up into a 4oz juice glass, dumping the old water and filling with filtered water and returning the fish to the bowl. If you get a betta you can get a male, a female or a male/female couple (never more than one male), you will still need to refresh the water from time-to-time to clear the waste product from the tank. If you want more fish or different fish than that, you’ll need the tank, filter, possibly pump, possibly heater setup.
