Anyway here is some suggestions, read my descriptions carefully as some are species only, and/or do actually school.
Have fun reading!
Scarlet Badis (Dario dario): The scarlet badis is the hidden gem of the aquarium world. Rarely found in pet shops, the shy scarlet badis blossom in nano tanks. Their timid nature makes community setups a poor environment for these fish. Species only tanks are preferred, with one male badis with a harem of females. Males are known to be aggressive towards each other, so tanks larger than 10 gallons are required if housing more than one male. This allows them to establish their own territories and females of choice.
Heavily planted tanks with subdued lighting and dark substrate help scarlet badis feel more comfortable in their home. Care must be taken to ensure each of the badis get enough food, especially if kept with other species, as they will not compete for food.
Max size: 1 inch
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Pygmy Sunfish (Elassoma sp.): The pygmy sunfish is one of the lesser known nano fish in the hobby. These fish enjoy a heavily planted tank with slow moving waters. Although not classified as part of the species, males exhibit cichlid like behaviors during spawning. They spend their days dancing and impressing females, and will fiercely protect eggs. The pygmy sunfish is somewhat difficult to feed, as they mainly accept live and frozen foods. Having a continuous supply of live foods in the tank allows the sunfish to scavenge through out the day.
Max size: 1 inch
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Endler’s Livebearer (Poecilia wingei): Endlers are quickly becoming more and more popular candidates for nano and community aquariums, and it’s easy to see why. These fish are an excellent species for new and experienced owners alike. Male endlers have stunning, incredibly vibrant colors that can be found in various patterns. Like their larger guppy cousin , endlers are known for being prolific breeders. Because nano tanks lack space for additional fish, it’s better to keep all males. Endlers are extremely adaptable, and very entertaining to watch. They tirelessly explore all levels of the aquarium, and add a flash of color and life to the room.
Max size: 1.5 inches
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Celestial Pearl Danio/Galaxy Rasbora (Celestichthys margaritatus): In 2006, an image was released of a stunningly beautiful freshwater fish discovered in Southeast Asia. In fact, it was almost too gorgeous to be real. The celestial pearl danio is a tiny fish that is quickly becoming a sensation in the aquarium hobby. These active little shoaling fish are renowed for their incomparable coloration, displaying rich reds, deep blues, and vibrant yellows when comfortable in their home. A heavily planted tank is crucial for this species, not only for cover, but to break up the lines of sight of rambunctious males. Male celestial pearl danios are known to have quite a temperment, constantly sparring with rival males over territory and females. Because of this, its best to house more females than males, to prevent weaker males from being harassed to death. The celestial pearl danio is also one of the few fish on this list that thrives in alkaline water.
Max size: 1 inch
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Boraras Species: The genus Boraras contains six distinct species of petite tropical fish; B. maculatus (The dwarf or pygmy rasbora), B. micros (the dwarf Thai rasbora, or three spotted dwarf minnow), B. urophthalmoides (the sparrow or exclamation rasbora), B. merah (the Phoenix rasbora), B. brigittae (the mosquito, chili or red rasbora), and B. naevus (the strawberry rasbora). All of these bright colored fish stay less than an inch long, and because of their diminutive size, they are often found as prey for larger fish. It’s extremely important that you keep Boraras species in groups of six or more, to ensure they feel safe and secure. Most of the less common Boraras Species are wild caught, making them sensitive to changes in water parameters. Very soft, acidic water is a must. Luckily, the tiny members of the Boraras genus are adaptable when it comes to food, and will learn to take micro flakes or pellets.
Max size: ~1 inch
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Shrimp: Although not a fish, shrimp are certainly the best choice for stocking a nano tank and it would be a crime not to include them on this list. Shrimp are very low maintenance, and incredibly easy to care for. They have a minute bioload, and you could easily keep a small colony of shrimp in a 2.5 gallon tank. Shrimp come in a huge range of colors and patterns. Ghost, crystal, and cherry shrimp are among the most common shrimp species. They can be housed in a single species aquarium, or with other peaceful nano fish. They’re great to have and they work tirelessly as a “clean up crew”, scavenging for left over food.
Max size: depends on the species, but most stay around 1-2 inches
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Lyretail killifish are a popular choice for smaller aquariums. If your tank is long and relatively shallow, you can keep a pair or trio in 8-10 gallons (30-38L), provided the setup is very densely planted.
Unlike many other killifish species Aphyosemion australe is not annual, which means they live for more than a year and you don’t have to breed them every year to keep your population intact. In fact, they can live for up to three years and are relatively hardy compared to other killies.
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