That's why under the sea, most fish no matter what breed/species live close to the coastal areas of the Earth in relatively shallow waters.
Even to get vegetarian food they must swim deep to the ocean floor- again in shallow waters. The same applies to fish in an aquarium.
If you have fish that are small like goldfish, then place them in an aquarium not more than one foot or meter deep as they'll have difficulty reaching the bottom to search for dropped food.
An aquarium this size should be suitable with not too many fish:
Also avoid putting fish in a bowl as they have limited space to move around. An aquarium would be more desirable. There is also the concern of the aquarium being too small. If you have fish that are large such as those that people eat, then get a larger aquarium that is deep and spacious.
