Is it harmful to let a goldfish swim in a round bowl?

Answer

Contrary to the general assumptiong, a jar is certainly not suitable for holding a goldfish, because a jar is usually much too small and does not have filtration.

A goldfish is a cold water fish and such fish produce quite a lot of waste, mainly ammonia. In too high concentrations (1ppm is already way too much) this poisons the fish, inevitably resulting in death. If you want to keep a goldfish, you should therefore count at least 40 liters of water per fish and have a biological filter that does at least 10 times the volume of the aquarium per hour. The biological filter ensures that ammonia is converted into nitrite (still toxic to the fish) and later into nitrate (which is only harmful at fairly high concentrations (50ppm)). Although 40 liters of water seems to be a lot, nitrate accumulates relatively quickly to harmful concentrations, so that at 40 liters per fish it is usually necessary to replace about 50% of the water once a week. If you don’t do this, the goldfish will suffer greatly, as a result of which he/she will not live long and will have a very early death (15 years old is no exception for a well-cared-for goldfish).

If you also take into account the fact that goldfish can become quite large (30 cm body length is not unique), then even that 40 liters can be far too little after a while.