In the earliest days of the fish keeping hobby, many people kept their aquatic animals in glass bowls for display. Unfortunately, this environment was oftentimes detrimental to the fish. However, to this day fish bowls and similar undersized and poorly shaped containers are still sold to beginning aquarists. This is essentially setting someone up for failure, as few fish will survive in such containers and to make it work requires a high level of skill and expertise. Although the stereotypical goldfish in a bowl has fallen out of favor in recent years, the betta fish is still commonly kept in tiny tanks with no aeration or filtration. Here we will explain why you should avoid fish bowls, vases, and other small decorative containers for your fish.
Fish bowls usually have very small openings at the top, so when they are completely filled, there isn’t much room for gas exchange. While this isn’t necessarily a problem for anabantoids and other fish that take in atmospheric air, this can be very bad for fish that require high dissolved oxygen content. Adding an airstone can help, but you’ll still run into difficulties with regard to the nitrogen cycle because...
In the smallest of bowls, there really isn’t room for much by the way of filtration. In order to make this setup work, frequent large water changes are necessary. This can be stressful on fish, however. Fish really thrive in an environment where there is a stable and healthy biological component. Without this filtration via an established nitrogen cycle, ammonia can build up in a bowl to toxic levels. Frequent water changes can dilute this, but again it is very stressful on the fish and with such a small volume of water, conditions can foul very quickly. Nitrifying bacteria will not survive without aeration, so if your fish bowl is stagnant there likely won’t be any beneficial bacteria activity.
Small volumes of water tend to go up or down in temperature rather quickly. Fish and other aquatic animals do not fare well when there is this constant fluctuation. Although some small bowls can accommodate heaters, these can just as easily cook fish in such environments. It’s much better to get a bigger aquarium so that the temperature changes more gradually and so that larger, adjustable heaters can be utilized (most small heaters are preset and therefore hard to control).
For bowls that are a gallon or less in size, there simply isn’t enough room for most fish to exhibit natural behaviors. This will stress the fish unnecessarily and may cause premature death. In the case of goldfish, who have long been the poster fish for keeping in bowls, they usually end up stunted from poor water conditions if they survive at all. It should also be noted that goldfish can live decades, but in a bowl they might only make it a handful of years if they're lucky. That’s nowhere near the quality of life such an animal deserves. Most fish, unfortunately, will not thrive in these conditions. So if you’re thinking of keeping fish in bowls, you may want to reconsider.
While some people may get away with keeping fish in a bowl for a few months or years, it is not the most ethical way to house them. More often than not, it is a recipe for disaster when starting out with keeping fish. Due to poor oxygen levels, no filtration, dramatic temperature swings, and severe restriction of movement, bowls are not a good choice for fish. We recommend starting out with no less than a 5-10 gallon aquarium for small community fish and bettas.
Beginner's Guide to Successful Fishkeeping from Marineland - http://www.marineland.com/information/infographics-and-guides/a-beginners-guide.aspx