*bowl refers to all filterless stand alone fish keeping containers
A goldfish in a bowl... see anything wrong with this picture? Most people probably don't. Keeping fish in bowls is cruel. With the exception of betta fish no fish should ever live in a bowl. But keeping a betta in a bowl is going to require special care.
Reasons why bowls are bad:
too small
no filtration
open tops leave fish vulnerable
A tank that is too small for your fish is a really bad thing. Sure, that little goldfish in the picture seems to fit well enough now but goldfish all grow to a minimum of 6 inches, some grow to be 2 feet! It's a myth that a fish will adapt in size to its environment and only grow to suit the tank. Fish like all other animals have their size determined by their genes. Forcing them to live in a home that is too small puts so much stress on them. A fish in a bowl will have stunted growth and die prematurely. Goldfish can live a very long time, some have been known to live as long as 20 years. The fish keeper's rule is 1 inch of fish per gallon of water, but for goldfish it is recommended that they have a little more space. No less than 10 gallons per goldfish, some say no less than 20 gallons. With fish bigger is always better, they like to have room to swim, play and explore just like we do.
Remember that fish as basically living in their own toilets. Waste from the fish themselves and uneaten food fall to the bottom of the tank and decompose. These decomposing things deteriorate the water quality which leads to fish death. If you keep a fish in a home with no filter you must change the water at least once a week. Note that water changes but a lot of stress on the fish. The tank is a fragile ecosystem, taking out the old water and putting in new water disrupts this. The old water contains beneficial bacteria and has an established nitrogen cycle. This is why experienced fish keepers and aquarium shops tell you that you must run a complete tank setup for 1-2 weeks at the very least before adding fish. A filter is really a necessary thing for a fish tank. Many people don't want to deal with filters because they're "too much work", but filters are very simple to use and much less work than having to very carefully change the water frequently.
Open tops leave fish vulnerable to things falling in their tank, other pets you may have and they may jump out and get hurt/ die. A fish tank should always have a cover.
Fish are living creatures capable of feeling pain, your pet fish needs you to do almost everything for them. If you love your pets, if you love animals, if you have a fish, please say NO to bowls!
