Contrary to popular belief, very few fish can survive in a fish tank, and even those that can need a larger space to really thrive. Neon tetras, despite their small size, need more room than a small bowl can provide.
Schooling and Stocking
In the wild, neon tetras school for protection from predators. Due to this adaptation, neon tetras feel unsafe and stressed outside of groups. In captivity, you need to keep neon tetras in groups of at least eight to keep them from feeling stressed. Aquarium stocking guidelines say you need at least a gallon of aquarium water per fish -- that's a minimum of 8 gallons to keep eight neon tetras comfortable. That's larger than a bowl.
Limitations of Bowls
Bowls have a number of serious limitations compared to full aquariums. For one thing, most bowls have curved lips, and aquarium filters are built to hang on the edge of rectangular aquariums. Their small size makes it hard to attach the filtration equipment, heaters and other aquarium equipment that fish like neon tetras need to thrive. Additionally, a bowl's small size precludes keeping a group of neon tetras together.
Despite their small size, neon tetras require a tank of at least 20 gallons to really do well. Neons need room to swim because they are very active fish. Additionally, neon tetras need clean, clear water and tropical temperatures -- think 76 to 82 degrees Fahrenheit. Sudden changes in temperature can cause health problems for fish. In a wee bowl, it's very difficult to maintain good water quality and avoid sudden changes in temperature due to the relatively small volume of water.
Nano aquariums are a new trend hitting the aquarium industry. In these tanks, experienced aquarium hobbyists keep very small aquariums or bowls. Such tanks take a lot of experiences and know-how to maintain. With lots of expensive, miniaturized equipment, nano setups can present stunning displays. Even then, neon tetras do not make an ideal addition to these tanks since they need lots of room to swim.