If you’re looking for the best fish for a small tank without a heater, then you’re in luck because you have a lot of options. In the following article, we’ll review the eight best coldwater fish for small tanks that are readily available on the market.
You might also want to try out our Aquarium Heater Size Calculator (Click Here) to see what size heater you would need for your aquarium.
Best Fish for a Small Tank without a Heater
| Image | Name | Temperature | Rating | Shop |
|---|---|---|---|---|
 | GloFish (Danio) | 64°-75° F |  | |
 | White Cloud Minnow | 64°-72° F | | |
 | Guppy | 64°-82° F |  |
GloFish (Danio)
GloFish (Buy Online) are a unique freshwater fish that can tolerate a wide range of water conditions. Their hardy nature makes them the perfect first fish for children and beginners. And their bright vibrant colors can even rival the colors of fish found in saltwater aquariums.
GloFish can handle water temperatures that range from 64-75 degrees Fahrenheit. As long as you can keep them within this temperature range they won’t need supplemental heating. One caveat is that GloFish will be more active if the temperature is above 72 degrees Fahrenheit.
GloFish 5 Gallon Crescent Aquarium Kit
GloFish are perfect for small tanks since they are a relatively small fish that only reach a maximum size of about 2.5 inches. If you try to follow the recommended stocking density of one inch of fish per one gallon of water. You will easily be able to keep a small school of up to five GloFish in a 10-gallon aquarium.
White Cloud Minnow
The White Cloud Minnow (Buy Online) is a small fish that is very tolerant of cold water. They are also very easy to care for since they are not picky about water conditions. And they will brighten up a tank with their golden bodies and vibrant red markings.
The white cloud minnow is very well adapted to living in cool water conditions. This stems from the fact that they are native to the Chinese Cloud Mountains. At those high altitudes, temperatures remain cold for most of the year. White cloud minnows are able to comfortably live in temperatures ranging from 64-72 degrees Fahrenheit. And they will breed and remain active, even when they are kept at the low end of that temperature range.
White Cloud Minnows will be much more active if they are kept in schools of at least five or more fish. And their color will even brighten up when they are around each other. And white cloud mountain fish are a small fish that don’t grow much larger than 2 inches. So even if you have a small tank, you will be able to keep a school of these very social minnows.
Guppies
Guppies (Buy Online) are one of the most well known colorful freshwater fish. They are usually the first fish people get when they are just starting out in the fish keeping hobby. And that is for good reason since they are a very versatile fish that can survive in a wide range of conditions.
Even though guppies are a tropical fish they can handle water temperatures that range from 64-82 degrees Fahrenheit. This wide range of acceptable temperatures means you can easily keep a guppy in an unheated aquarium. Just be aware that if you want to breed your guppies, they will only breed when the water temperature is above 77 degrees Fahrenheit. And as with most tropical fish, they will be more active and playful at higher water temperatures.
Keeping guppies in a small tank should not be a problem. Guppies are a small freshwater fish that reach a maximum size of 2.5 inches. If you have a small tank you should be able to keep a few guppies without running into any problems. As usual, just try to follow the rule that one inch of fish requires one gallon of water. If you stick to that rule it will prevent you from having any of the problems associated with overstocking an aquarium.
Platies
Platies (Buy Online) are easy to care for fish that add color and activity to any aquarium. These attributes make platties a great first fish for beginners. They also give birth to live young, so they are the perfect fish to introduce someone to the fun of fish breeding.
Keeping platies in an unheated tank won’t be a problem since platies can handle water temperatures that range from 64-77 degrees Fahrenheit. As long as the temperature in your tank remains relatively stable and doesn’t swing wildly then platties will do just fine. But if you do want to breed your platties they will need to be kept in water that is above 77 degrees Fahrenheit.
Platies are small fish that don’t grow much larger than 2 inches. Platies are also more tolerant of cramped conditions then similarly sized guppies. And in a worst-case scenario, they can handle being in an overstocked tank since they are very tolerant of poor water conditions. You should of course always stock your small tank appropriately, and try to avoid having any more than one inch of fish per gallon.
Endler’s Livebearer
Endler’s Livebearer is an extremely hardy small freshwater fish that not many people are aware of. Endler males are beautiful fish that exhibit hues of orange, green and blue all across their body. They are so colorful they even make a great substitute for neon tetras, especially in a cold water aquarium.
Endler’s are native to northern Venezuela where they live in shallow streams and lagoons. These shallow bodies of water can experience wide swings in temperature and water quality. Because Endler’s evolved in those conditions, they can live in water temperatures that range from 64-82 degrees Fahrenheit. And they will breed and remain active even when they are kept at the low end of that temperature range.
Endler’s are a very small fish that only reach a maximum adult size of 1 inch. Their small size makes them a perfect fish for a small tank. They do like to swim around a lot so they will appreciate a large tank if you can provide them with one. But if all you have is a five or ten-gallon tank you will still be able to have a nice school of Endler’s.
American Flagfish
The American Flagfish is a very easy to care for Killifish that can thrive in very small tanks. Since the American Flagfish is a Killifish they are able to breath atmospheric oxygen. That allows them to live in very cramped conditions just like Bettas.
The American Flagfish is native to Florida where they live in shallow swamps and marshes. And for short times they can even live on land if they have to. This wide range of adaptability allows them to survive in temperatures that range from 66-72 degrees Fahrenheit. But they are more active when the water temperature is at least 70 degrees Fahrenheit.
Since American Flagfish live in swamps, they are used to living in small bodies of water. Sometimes they even have to live in a puddle for short periods of time since swamps can experience rapid water level changes. And they only reach a maximum adult size of 2.5 inches. And to top it all off they can survive in water that has a very low concentration of dissolved oxygen. When you put all of these things together you get a fish perfectly adapted to life in a small aquarium.
Mosquito Fish
The Mosquito Fish is a small fish that comes from the same family as guppies. And because of this relation, they have the same ability to adapt to different water conditions. And as their name implies they are voracious consumers of mosquito larva.
Mosquito fish can live in water temperatures that range from 65-75 degrees Fahrenheit. And they are very tolerant of poor water conditions. Due to their ability to handle poor water conditions they are even added to stagnant backyard pools to help control mosquito populations.
Mosquito fish are not the smallest fish on this list, so you might not be able to keep as many in a small aquarium. An adult mosquito fish can reach a size of up to 3 inches. That might mean you will only be able to keep a few of them in a small tank. Or you can keep a small school if you have a tank that is close to twenty gallons.
Comet Goldfish
Comet Goldfish (Buy Online) are the smallest and easiest to care for of all the goldfish. And you may have even seen these fish being given away at a local carnival as a prize. When most people think of a pet fish the Comet goldfish is usually the first fish that pops into their mind.
Goldfish are one of the most cold water tolerant fish available. They can easily handle temperatures that range from 65-75 degrees Fahrenheit. And some goldfish even survive outside in ponds throughout the winter. They have even evolved to enter into a state of quasi-hibernation to survive such extremely cold temperatures. But in a tank that stays at least above 65 degrees Fahrenheit they will remain active year round without having any problems.
The only downside of keeping a Goldfish in a small tank is that they will eventually outgrow it. At first, they will be small so they will easily fit in a small tank. But eventually, they will grow to a size of up to 12 inches, which is way to big for a small aquarium. If you only plan on keeping a goldfish in a small tank when they are young this won’t be a problem. As they get older though you will have to upgrade their tank since they will eventually outgrow the tank they started in.
