If You Must!
Here, for those of you who can't resist, are some thoughts on keeping fish in bowls.
Even in a bowl, fish need some security. Always have a 1" layer of gravel in the base. This not only gives the fish a reference point, it also provides a base for friendly bacteria which help dispose of fish wastes. Some sort of ornament or plastic plant will also help the fish by providing a little bit of shelter.
Never keep more than one goldfish in a bowl. They do not get lonely and if you feel that your fish MUST have a friend then get a tank and filter. Ideally each 1" of fish should have 30 sq." of water surface - and goldfish can grow to 10-12"!
†There is in fact ONE bowl that I feel is an acceptable home for fish. Itís called the Biorb, it holds thirty litres of water and has a built in filtration system.
Bowls and unfiltered small tanks require 50% water changes weekly. The water should be de-chlorinated and conditioned (normally one product) before being added to the tank or bowl. It should also be left to stand so that the temperature is equalised. If the bowl or any of the contents need cleaning it is best to remove 1/2 the water and the fish, then clean the bowl and/or contents using the remaining water. Never use untreated tap water as this contains chemicals like chlorine to kill off biologically friendly as well as unfriendly bugs. Using 'Cycle' or some other biological booster or culture will also enhance the missing bio cycles like the nitrogen cycle.
Bettas are TROPICAL fish. They should ideally be kept at a steady 75F. Very few bowls can provide this.
There are a few semi-tropical fish that will survive lower temperatures. White Cloud Mountain Minnows, Zebra danios and Bronze cory catfish will generally be fine as long as the temperature does not drop too low. These fish all prefer to shoal though - again a tank is a better option.
Be very frugal with feeding your fish. Never give it more than it will eat in two minutes, once a day.
Be aware that bowls have very small volumes of water and so are subject to much greater variations in conditions than larger tanks. Do all you can to minimise this.