I would think of much more reasons not to, but if not for the fish for ourselves then: Bowls are boring, dirty looking, and too tacky.
Here are five reasons why you should avoid using a goldfish bowl:
Goldfish bowls are too small.
Goldfish can easily grow to several inches in length with good care, and monster goldfish of over six inches are not unheard of. Now, imagine living in a room the size of your bed. That’s about the equivalent for a goldfish in a bowl. Not nice.
They can deprive goldfish of oxygen.
Goldfishes need oxygen from the water, just like we need oxygen from the air. Too little oxygen and they become stressed, and may even die. One of the things that affects how much oxygen is in the water is the surface area, which is where the water meets the air. You can think of oxygen crossing over into water like fans at a football game going through a turnstile. A lot of fans and only a few turnstiles is a recipe for slow turnover. Similarly, a small surface area with a bigger volume of water below (caused by the bulging of the bowl) means not enough oxygen is getting through for your pet.
Goldfish bowls are boring, for your fish and for you.
A goldfish bowl is too small to get much decoration into, so the fish is lucky to get some gravel or marbles and a pebble or two. Soon you’ve got a fish too bored to swim and an owner too bored to care. Not a recipe for healthy goldfish.
Bowls get dirty too quickly.
Some new kinds of bowls include filters, but the traditional goldfish bowl isn’t compatible with the technology that can keep an aquarium clean. Result: dirty water and gravel. Admittedly goldfish bowls are quick to clean, but only because they’re so small, and we’ve already discussed why that’s bad.
You can only keep one goldfish in a bowl.
You might get by with a couple of inmates for a while, but sooner or later you’ll forget to do a water change or overfeed the fish and the bowl will pollute too quickly. With just one goldfish in a bowl this problem is somewhat reduced, but goldfish are friendly, naturally shoaling fish that prefer the company of their fellows. It’s cruel to keep them alone.