Fish

I'm going to assume that an unheated tank will stay mostly within 65-75 degrees Fahrenheit, and not change temperatures too rapidly.

If you have a large, well-filtered tank, you can keep goldfish (about 25 gallons per fish is the minimum for the short-bodied fancy varieties; common goldfish, shubunkin, and comet varieties need more). Goldfish are very messy fish that require heavy filtration and well-oxygenated water. Even then, significant weekly water changes are needed. Figure on changing about 25% or a quarter of the water every week.

Goldfish also love to eat plants, so the only suitable ones for goldfish tanks are 1) very tough, 2) very bad-tasting, or 3) small floating plants that reproduce so quickly they outpace the goldfish's appetite. Tough plants include Anubias and Java Fern. Java Fern and Java Moss are also bad-tasting to goldfish and will mostly be left alone. Duckweed is a floating plant that filters the water of toxic ammonia and nitrates (a bonus for messy goldfish), and reproduces so quickly that it can take over a tank in days if not constantly removed or eaten. The best part is that most people consider it a persistant weed in their tanks and will gladly fork over "spares" for you to use. If you find yourself with more duckweed than your goldfish can eat, be sure to compost it or throw it in the trash so that it doesn't end up choking your local waterways. Duckweed, Java Fern, Java Moss, and to a lesser extent Anubias (which grows slowly) will all help keep the water free of toxic fish wastes by turning it into plant tissue. Besides, brightly colored fish look best against greenery!

For tanks 10 (US) gallons and up, White Cloud Mountain Minnows are an excellent choice. They require a small school of at least five to reduce stress and show their best colors. These are good fish for a community if you have a tank larger than 10 gallons, otherwise keep the minnows to themselves as small tanks like this are difficult to maintain.

Peppered Corydoras catfish prefer cooler water than most of their tropical relatives and do well in unheated tanks. Like most other corydoras, they are excellent community fishes and bumble around peacefully at the bottom, looking for meaty foods in the gravel or sand. Make sure to use a rounded or very fine gravel otherwise their barbels can be rubbed away and get infected.

Bloodfin Tetras will do fine in the typical unheated tank that hovers around the high sixties or low seventies, and it exceptionally hardy for a tetra besides being colorful with a white-clear body and bright red spots near the anal and tail fins. Like most tetras, they need to be kept in a small school of at least five.

Buenos Aires Tetras are hardy for cooler temperatures into the high sixties, but they are not very friendly to some other fish or fine-leaved plants. They are, however, very colorful and tough fish. Cory catfish should be fine, and larger or faster tetras and danios without flowing fins might be safe. Aggression is less likely when they're kept in a group of 6-8, as these are social like most tetras. They'll tend to focus their antics on each other rather than tankmates.

The wild variety of Gold Barbs, called the China Barb, is pretty cold-tolerant and will be fine in a tank that's between 65-75 degrees Fahrenheit. Like most barbs, these fish are both very active and need company of their own kind, so plan on housing at least five together. It's best to keep them in a 30 gallon or larger tank, they need the room to swim around.

Rosy Barbs are a shiny pink-to-red and sometimes greenish fish that will do fine under the same conditions, but they can grow about 1 inch larger than Gold or China barbs, so you may need to reconsider the stocking ratio of other fish or buy a larger aquarium to fit them all in comfortably. If you keep too few Rosy Barbs together, they might harass other fish and nip fins.

Zebra Danios are small, flashy, active fish that need to school with at least six of their own kind in a tank that's longer than it is tall, with a 20 gallon "Long" or bigger being ideal rather than a 10 gallon aquarium. They're common, inexpensive, hardy fish that have been in the hobby for longer than anyone can remember, and always popular. There are several man-made varieties that have been selectively bred for long, flowy fins or leopard-spotted patterns. Although pretty cold-tolerant, they do best in water ranging in the 70s F.

Japanese Ricefish or "medaka" are tiny, cream-colored, egg-laying fish that look especially good among fine-leaved plants. It may be difficult to find true Japanese rice fish, as some closely-related fishes from other parts of Asia are often misnamed and offered for sale. They're so hardy that, like goldfish, they've been popular pets in Japan for centuries, before all our modern filtering and heating techniques were invented.

For a tank of at least 20 gallons or larger, Dojo loaches (aka "weather loaches) are fine bottom-dwellers with an interesting habit of becoming much more active when the weather changes. They like water of neutral or slightly basic pH, and like Cory catfish need a substrate that won't wear down their barbels. Like many loaches they prefer to be in groups of at least three, they're very gregarious animals. They also like to burrow in the substrate, or hide under plants and objects; they feel most secure if there are plenty of hiding spots. Make sure any rocks, wood, etc. are free of sharp edges, as this is a scaleless fish. They don't tolerate salt in the water for the same reason, so be careful of that. If you're prepared to have a group of large, active, elongated fish then these are for you. Take the right precautions and this is an extremely hardy, long-lived fish. As a bonus, they do well with goldfishes and provide an interesting contrast in body shape and swimming habit if you have a spacious enough tank.

Many fishes native to North America are at home in unheated tanks, and some of them are even colorful or interesting enough to rival tropical species. Golden Topminnows are a hardy native killifish that live up to their namesake for color. Rainbow and Red shiners are colorful minnows that get bright red and blue patterns in the males, especially during courtship. Though not always suitable for a community tank, American-flag fish are small green pupfish or killifishes where the male develops a deep, dark patch on the back and bright red stripes along the sides, making it look remarkably similar to the American flag. They also like to eat some types of algae. Dwarf Livebearers/Least Killifish are tiny little guppy-like fish with green and black colored bodies, usually with some orange on the dorsal fins. They're hardy, peaceful little fishes that enjoy heavily planted tanks and won't eat their own young, but spawn almost as readily as guppies. The main problem is that they're so small they can't be housed with many other types of fish who would consider them to be snacks.

"Rosy Red" or fathead minnows are good community fish and hardy in a wide range of temperatures. They certainly get along fine in an unheated tank. Aside from looking like young goldfish, they also have cichlid-like spawning behaviors where the male guards the clutch of eggs until the fry are free-swimming, which makes them more interesting than real goldfish.

A word of warning on rosy reds, though. They're often sold as "feeder" fish which is a mixed bag: one the one hand, they're very cheap and you can get a dozen for a few dollars or less. On the other hand, these fish are often grown in overcrowded conditions and might carry diseases that shorten their lives or infect other tankmakes. It's best to keep these fish in a quarantine tank for a couple of weeks to observe them for signs of disease. That way you can easily treat them if problems show up. Generally you shouldn't have problems but it's always best practice to quarantine fish before moving them to their final home.

In North America "native fishkeeping" is a niche hobby to itself, while some of the species mentioned above are more popular with general aquarists in Europe.

Paradise Fish are perfectly happy in unheated tanks, but they are not good community fish and are even more aggressive than male Bettas. Best kept singly. They might get along with corydoras-type catfishes. Generally speaking larger, faster, less-colorful fish without flowing fins won't be attacked, and you should also consider whether the fish in question is liable to nip the fins of the paradise fish.

On the plus side they're large, hardy, and colorfully-striped fish that can be kept in tanks as small as 5 gallons if you stay on top of water changes. They're more suitable for small, unheated tanks than their more common competitor, the Betta.

One fish that is often sold for unheated tanks (or jars) are Bettas, but these fish actually prefer very warm water around 80 degrees F. Cooler water makes them sluggish and more vulnerable to diseases.

This is a list of commonly available, colorful species that are generally hardy and easy to maintain in an unheated tank, but there are many more waiting in the wings beyond the rows of "tropical" fish. Of all the ones you're likely to encounter in a pet store, these are probably the most widely sold: White Cloud Mountain Minnows, Zebra Danios, Rosy Barbs, Peppered Corydoras, Bloodfin and Buenos Aires Tetras, and of course Goldfish.

Rosy Red Minnows are often sold small as feeders for bigger fish or some reptiles, as are drab-looking guppies or mosquito fish (be careful of these, they can be aggressive for their size), and many stores carry Dojo Loaches (often the "golden" color variety). Gold/China Barbs and Paradise Fish might be hit-or-miss. Very few stores will keep stock of Medaka, and in the US you're much less likely to see any native North American fish on display unless the owner is an enthusiast, or unless they sneak into shipments of "feeder" fish.