Fish, like all animals, need a certain amount of space and they really don't appreciate goldfish bowls. Thousands of goldfish are given out at fairs and carnivals each year to die in a few days. So goldfish bowls are all over the place.
However, as many above have already stated, Bettas are probably the most hardy things you could keep since they live in stagnant canals from SE Asia. They are part of a group called Anabantids and have specialized gills that allow them to gulp air and therefore survive low oxygen conditions.
But they are tropical fish and need room temps of about 75 degrees to be happy so sitting them on a windowsill means they get all sorts of extremes.
Goldfish, on the other hand are very cheap and tolerant of cold. I keep them outside in a pool all year and they grow about a foot long and breed every spring. They are just a form of Asiatic carp and easily survive a temperate winter. But in a bowl they rapidly get stunted and tend to be very dirty. All freshwater fish have hyperactive kidneys to get rid of the constant influx of water that invades their cells (a hypotonic environment). Therefore, in just a few days or so, a goldfish bowl becomes a sewer and the fish is swimming in essentially urine in a very short time. Therefore they need frequent changes of water to be healthy and most people don't exercise that kind of level of care for such cheap fish.
To keep fish well requires work. They aren't toys.
Hope this helps.
WC
