Certain fishes are often kept in bowls or small plastic aquariums that lack heating and filtration equipment. The fish most often kept in this way include goldfish, bettas, white cloud mountain minnows, and danios. Each of these fishes have different pros and cons when it comes to keeping them in this fashion.
Goldfish, Carassius auratus: Superb in Large Unheated Aquariums
Goldfish grow. Common goldfish can grow up to approximately 12 inches in length which is larger than some of the containers the juveniles are kept in. They also produce a good deal of waste as they grow. Fancy varieties usually stay smaller than common goldfish, but their fancy fins are inclined to develop finrot in dirty water. This makes the lack of a filter problematic. Goldfish are cold water fish so the lack of a heater is perfectly fine.
In short, they are ideal for a large unheated aquarium with filtration but are not suitable for a bowl.
White Cloud Mountain Minnow, Tanichthys albonubes: Good for Small Unheated Aquariums and Large Bowls
White cloud mountain minnows prefer lower temperatures than most tropical fish and for this reason they are often kept in unheated aquaria. Their natural temperatures are 64-71F. They are very small fish, staying under two inches in length. However, they do school so is important to provide enough space for at least a couple of fish, preferably more. Fortunately their small size means that two or three can be kept comfortably in a 2 or 3 gallon aquarium. Of all the fish discussed here, these are probably the best for a large unheated bowl or small unheated aquarium.
Zebra Danio, Brachydanio rerio: Not Ideal
Most danios are small, the zebra danio being 1¾ inches in length, and they are tolerant of a wide range of temperatures. Their natural temperature range is 64-75F. However, they are fast swimmers and like to school which makes keeping them in a tiny aquarium questionable.
Watching a school of these fish in a large tank is a completely different experience from watching one or two in a tiny tank. In a large tank they can get up speed and they seem to fly through the water in a group. It is beautiful, but it is something you will never see if you put these fish in a betta cube.
Betta, Betta splendens: Ideal for Small Heated Aquariums or Bowls
Bettas stay small, usually growing to approximately 3 inches in length. The waters from which they come are naturally rather stagnant and they have the ability to take in oxygen from the air. This greatly increases their resistance to foul water. They dislike fast filtration. They are solitary in temperament and swim slowly. This makes them seem like an ideal candidate for the small unfiltered bowl.
However, the waters from which they come are warm: the average temperature is 24-30C (75-86F), according to Fishbase.org. While they will survive in an unheated bowl provided the room is decently warm, keeping them in an unheated aquarium seems questionable.
Some of the object in which Bettas are kept are too small for any fish. They should have room to turn around easily and room to spread those magnificent fins of theirs. Keeping these fish in an adequate sized and warm tank has advantages to the fishkeeper in addition to feeling good about the way you keep your fish. Healthy fish display more, build bubble nests and check every nook and cranny of the aquarium for missed food. They are much more interesting to watch, and a betta doing an aggressive display in a well lit aquarium is a truly stunning sight.
It doesn’t take that much effort to set up a small aquarium with a heater, and this fish will reward you well if you do. Secondhand equipment can often be bought for less than new equipment and a 2 gallon aquarium is more than enough space for a Betta.
Least Killifish, Heterandria formosa: Ideal for Small Aquariums or Bowls
This is a very small fish native to North America which is related to the guppy. The Males are extremely small at 3/4 inches, and they prefer normal tropical tank temperatures of 68-78F. Their small size and tolerance of moderate temperatures makes them ideal for a small unfiltered aquarium, provided it is not subjected to cold temperatures. The largest problem with these fish is likely to be finding them, since they are not common in aquarium stores.
Invertebrates: Better Than Fish for a Bowl
Freshwater invertebrates may work well in truly tiny tanks if you do not wish to keep fish. Freshwater shrimp are harassed by many fish species and may do better on their own. There are also many species of snail, and a shrimp-like creature known as Triops. Different species have different temperature tolerances, but some of them tolerate cool water very well. You could also try brine shrimp if you are willing to handle very salty water.
Note: All temperature information is taken from fishbase.org, and are the temperatures at which the fish have been found living in the wild. Some species can tolerate wider ranges.