If you have a small aquarium, it can be pretty hard to find the right fish to stock it with. Most of the popular choices for smaller tanks – pygmy corydoras, dwarf puffers, clown killifish – should still be kept in at least around 10 gallons (40L) and are wrongly marketed towards smaller setups. So what choices does that leave us with? Keep reading for a list of the actual smallest of the smallest fish!
Note: Please do not keep fish in any aquarium smaller than 5 gallons. These extreme nano tanks cannot hold a stable cycle and you should only keep invertebrates with a small bioload in them. Please also do not keep more than one species of fish in a nano tank to prevent overcrowding. This list contains some suggestions for suitable invertebrates.
Boraras brigittae – Mosquito rasbora
Talk about tiny! At a maximum of 2 cm (0,8 inch), these schooling fish – pictured above – are ideal for nano setups. They come from the soft, dark waters of Borneo, which means a great opportunity to set up a mini blackwater habitat. Keep mosquito rasboras in larger groups (at least 7-8) in aquariums of at least around 6 gallons (23L) – a longer aquarium is preferred here, because although they are small these fish do love to swim a lot.
Note: Mosquito rasbora are not the only ultra tiny rasbora. If they’re available in your local aquarium store, you could also consider Boraras maculatus (dwarf rasbora) or Boraras merah.
Heterandria formosa – Least killifish
Although Heterandria formosa are commonly known as “least killifish”, they’re not actually killis! These tiny livebearers are a fun species to keep in your nano tank due to how easy it is to breed them. A ‘harem’ with 1 male and 2 females can be kept in tanks as small as 5 gallons (20L), while bigger groups are possible in slightly bigger setups. Because they naturally occur in very slow flowing waters, least killifish don’t appreciate a heavy filter flow. Try finding a filter with a gentler flow (like a sponge filter) and go for dense plantation to keep water values stable and imitate the natural habitat.
Betta splendens – Siamese fighting fish
I’m sure you were all expecting this one, but I just have to mention them because they’re such a fantastic species. Bettas are often marketed as suitable for tiny unfiltered bowls and vases, which they’re unfortunately not. However, that does not mean they’re not a great choice for small setups. Heated, filtered aquariums of at least 5 gallons (20L) make a great single-fish setup for Betta splendens. They naturally occur in waters with dense vegetation, so consider live plants to imitate the natural habitat and provide your betta with a place to hide.
For more information about betta care, check out the Betta caresheet.
Wondering why exactly bettas can’t be kept in a bowl? Check out Betta than a bowl, where some common betta myths are smashed.
Although these schooling microfish were originally thought to be rasboras, they seem to be more closely related to the danio family. Their gorgeous colors make them a great centerpiece for your nano setup, although many sources report that they are quite fragile and may be picky eaters. Because of this, and the fact that they should be kept in schools and the males can be aggressive towards each other, I wouldn’t advise keeping them in the smallest possible nano tank, but instead going for something between 8-10 gallons (30-38L). This makes it possible to keep a slightly larger group and maintaining a good water quality will be easier.
These fish seem popular with shrimp keepers because of how extremely peaceful they are, so if you’re looking for an addition to your dwarf shrimp tank they may be a good choice.
I hadn’t actually heard of this fish until it was recently suggested by a reader, and I couldn’t believe they aren’t kept more often! These catfish are peaceful and stay very small, with a maximum of around 3,5 cm (1,4 inch), which means they’re definitely suitable for nano tanks. Although there isn’t much known about them, it seems that 3-4 specimens do well in a 8-10 gallon (30-38L) aquarium because they’re not too active. A carnivorous diet is preferred – frozen foods are great, but make sure they’re small enough for the fish to eat.
When stocking a nano tank, please always keep in mind that the welfare of your fish is more important than keeping as many species as possible. It’s quite difficult to keep the water values stable in smaller aquariums, so don’t overstock and test the water often. If you don’t have time to keep a close eye on your nano tank, don’t go for fish. Stick with inverts! There are many great species to choose from.
If you have any more questions about stocking and keeping aquariums under 10 gallons, be sure to leave a comment below. Happy microfishkeeping!