Is it okay to keep fish in a bowl?

Water currents

Some fish like more water movement, and some like a little, and some don't like any water movement at all! A tiny space like a bowl doesn't allow the fish to choose where they'd like to be in the current, and so predisposes them to stress. Fish are incredibly sensitive to water parameters, and they need choices to allow them to self-regulate their body functions.

Swimming space

Even though fish are small, they still need space! Carp, the wild ancestor of goldfish, can travel over 1 km in a day! Fish gain security from constant movement, as it allows them to find food and avoid predators, which means that fish that live in a constrained environment will automatically be more stressed out. Fish also need space to hide and to move away from unwanted stimuli, and there is nowhere to hide in a bowl!

Companions

Many fish require companionship, and small tanks or bowls don't provide enough space to keep multiple fish. Small fish like neon tetra and guppies are natural schoolers, which means they like to live in larger groups. Notoriously solitary fish like bettas still benefit from having tank mates of different species, such as shrimp and snails.

Enrichment

Fish need enrichment too! Think about the wild environment in which fish live - there are rocks, plants, sand, debris, and more! Compare that to a plain bowl, and the difference is clear. Fish have evolved to interact with a complex, constantly changing environment, and a tank needs space for furniture, as well as free swimming areas. Great options for tank furniture include driftwood, ceramics, and live plants.