Nano fish are typically a smaller fish that are kept in aquariums less than 20 gallons, although some aquarist would even say up to 30 gallons would make it a nano aquarium.
Here are my top 5 favorite nano fish:
Ember tetras are a small peaceful little schooling nano fish that only require a minimum of a 5-gallon tank to keep them in. Keep in mind that schooling fish are typically kept in minimum groups of 6, so if you can get bigger than a 5-gallon tank that is always a plus. These tiny little tetras only get to be about 1 inch in size and do best in temperature ranges between 73 and 84 degrees. So with such a wide temperature range, they can easily be kept with a lot of different species of fish.
pygmy cories are very small only getting to be about an inch in size. They do best within a pH range of 6.5 and 7.5, with a temperature range of between 72 and 78 degrees. While Cories are technically not considered a schooling fish, they do tend to thrive best in groups, and with the small size of the pygmy cory, I would still recommend to getting a minimum of 6 for your tank. The minimum tank size for these fish is going to be at least a 10 gallon, but of course, once again, bigger is always better, if it is something you can do.
Unlike the previous two, the Pea Puffer really should be kept in a species only tank. The minimum tank size for a single Pea Puffer is going to be 5 gallons, and you want to have an additional 5 gallons for every additional Pea Puffer that you are keeping in your aquarium. As I said, these fish tend to be known as being aggressive, so a species only tank is best. They are not really a good community fish. While they only get to be about an inch in size, they can be very territorial. So thus they need a lot of space. They do best with a pH of between 7 and 7.8, and they do not have a very wide temperature range on them, with only between 78 and 82 degrees. So if you live in a cooler climate, you will need to get a heater.
White Cloud Mountain Minnow
The white cloud mountain minnow is a peaceful Coldwater nano fish, that can handle temperatures between 60 and 72 degrees, the minimum tank size for these fish is gonna be about a 10-gallon tank. While they are of small size, only getting to be about 1.5 inches, they do like to swim. So 10 gallons or larger is perfect for them. now white cloud mountain minnow is a schooling fish, so you will want to get at least 6 for your tank.
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Sparkling gourami is considered a relatively peaceful fish. They will, however, sometimes if you have more than one male in a tank, there might be some aggression from male to male. These fish require a minimum of a 5-gallon tank, although again, I can never emphasize, bigger always better. These are a beautiful tiny little gourami only getting to be about an inch and a half in size. These nano fish also do not have a very wide temperature range, doing best between 77 and 83 degrees. So these are another one you will probably need a heater for. They also do well with a pH between 6 and 7.5 and they are a relatively hardy fish.