Aquarium fish do their best if you have a filter. Certain species have a reputation for surviving without one, but such species won't live their full lifespans or grow to their full size in unfiltered bowls. Some advanced hobbyists keep a few fish in large, unfiltered tanks, but this takes experience and a deep understanding of aquariums. Even then, the practice isn't without controversy. In short, keeping aquarium fish without filters isn't a good idea.
The common betta often gets promoted as a fish that does not need filters. However, fish kept in unfiltered bowls and vases rarely live their full lifespans. Bettas in bowls and vases may live an average of two years, while bettas in real aquariums may live longer than eight years. The biggest concern in a jar is temperature. Bettas hail from the tropics, and a room temperature in most of the U.S. and Europe will stress them out.
Paradisefish belong to the betta family, and have a lot in common with their tropical relatives. However, paradisefish come from subtropical and temperate regions, meaning they can better adapt to unheated aquariums. They also posses the ability to breathe air in stagnant water. While these fish are more suited to life in a bowl or vase than their betta cousins, they will still live longer, healthier lives in real aquariums with filtration.
Goldfish and Bowls
While a goldfish in a bowl is an iconic image of the aquarium hobby, this practice can harm fish. Experts recommend you give a goldfish at least 30 gallons of aquarium volume per fish -- with filtration. Goldfishes can survive in bowls or smaller aquariums. However, a healthy goldfish will grow to 8 to 12 inches in several years, and live for at least two decades. Without adequate room, a goldfish's waste will stunt its growth and dramatically shorten the fish's lifespan.
Some advanced aquarium hobbyists sometimes keep unfiltered aquariums as a challenge. Success in such an endeavor requires a deep understanding of many aspects of aquariums, including the care of aquarium plants. Hobbyists have to take great care not to overfeed, and must perform water changes even more frequently than usual. Even under these conditions, this kind of setup can only work with a few hardy fish in a very large aquarium.