In a bowl, the water quality is hard to maintain, as most are between .25g to 1g in size. Therefore making them a small size that cannot hold all of the essential equipment. But in a tank, you can fit in a filter, heater, thermometer, and you have more room to decorate the tank as it has a flat bottom instead of a curve. Tanks also come in a greater variety of sizes, which can be helpful when stocking with anything from neon tetras to common plecos. Unfortunately, a bowl does not have enough room to harbor even the smallest goldfish, which should be kept in a 20g+ tank, as the ammonia would build up and kill the poor thing.