Well, let me help you with that.
5-gallon aquarium tanks are best for single fish such as;
Betta fish (Betta splendens),
Least killifish (Heterandria formosa),
Cherry shrimp (Neocaridina davidi),
Dwarf crayfish (Cambarellus spp.),
Thai micro crab (Limnopilos naiyanetr),
Smaller snails,
Betta fish (Betta splendens)
Betta fish are excellent fish for stocking a 5-gallon aquarium: they’re beautiful to look at, don’t require much extra maintenance, and fit perfectly in a standard 5-gallon setup.
These beautiful fish do especially well in planted betta tanks stained with plant tannins and filled with natural hiding places. Of course, keep in mind that they are aggressive fish, and pairing them will result in constant fighting.
If you want to branch out and try something different, you can also try other wild species of betta, such as Betta smaragdina, Betta mahachaiensis, Betta imbellis, or Betta siamorientalis.
Just remember that these fish do best on their own and should not be kept in pairs except for breeding or if the species allows.
Least killifish (Heterandria formosa)
While you can keep a pair of least killifish in a 5-gallon nano tank, keep in mind that they may breed, leading to overstocking. In this case, you’ll need to have larger spaces available for growing out the fry and eventually rehoming them.
Otherwise, least killifish will also appreciate a densely planted 5-gallon aquarium.
Cherry shrimp (Neocaridina davidi)
If you don’t want to have fish in your aquarium, then a shrimp tank is the next best thing. Cherry shrimp are relatively easy to maintain as long as water parameters are kept in check. They bring incredible pops of bright color against a green backdrop!
If you’re lucky, the adult shrimp will start to reproduce and quickly fill the tank on their own, which is good as they can be quite expensive. You may add 3-5 individuals to start.
In addition to cherry shrimp, you may also add orange sakura, yellow shrimp, blue fairy shrimp, and other variations. See this thread for more freshwater shrimp options.
Once added to the tank, ensure that your shrimp are protected from being sucked up by any filters or wavemakers.
Dwarf crayfish (Cambarellus spp.)
Dwarf crayfish are favorites for stocking nano setups because they are small but pack plenty of personality!
Crayfish can comfortably be kept in a 5-gallon aquarium as long as they are given plenty of hiding places.
When picking a dwarf crayfish species, be sure to choose one of the smaller ones, such as the least dwarf crayfish (Cambarellus diminutus) and not the larger Mexican dwarf crayfish (Cambarellus patzcuarensis) that needs more space.
At most, only two dwarf crayfish should be kept per 5-gallon aquarium. You will also want to check compatibility with fish species if you plan on transferring them to a larger aquarium eventually.
Thai micro crab (Limnopilos naiyanetr)
Thai micro crabs are so small you might not even see them! These crabs aren’t the most obvious choice for a 5-gallon aquarium, but they can bring some life to a small ecosystem.
Thai micro crabs are perfect for nano setups that have lots of hiding places and peaceful fish; this crab species can be quite rare to come across and are often recommended only for experienced nano tank keepers.
Otherwise, they are primarily scavengers and filter feeders that help remove algae and other microorganisms. They easily get along with shrimp and snails but might be eaten by more advantageous fish.
Smaller snails
A simple planted setup with a couple of freshwater snails might bring the peace to your home that you’ve been missing. Aquariums don’t need to be complicated to be beautiful!
Nerite (Nerita spp.) and assassin snails (Clea helena) are some of the smallest, most manageable snail species available. Nerite snails need brackish conditions to reproduce, which makes population control very easy; assassin snails lay one egg at a time and will go after each other if there is no easier prey available.
Because they’re carnivorous, assassin snails should never be kept with other snail species if you don’t want to control pest populations.
Planting
Although your options for fish are limited, 5-gallon aquariums are perfect as planted tanks. There are plenty of plant species that stay relatively small and don’t require much maintenance.
So you might be thinking, if the “golden” rule states that it is 1 inch of fish per 1-2 gallons of water how come small fish like guppies or tetras , which are some of the tropical fish recommended for 5 gallon tanks that weren’t on the list and for a few more which should never go in a 5 gallon tank
These species are either too active, grow too large, or cannot handle the unstable water quality that comes with a smaller volume of water.
THE BIG NO-NOs for a 5 gallon tank
Fancy goldfish
Common goldfish (Carassius auratus)
white cloud mountain minnows (Tanichthys albonubes)
celestial pearl danios (Danio margaritatus)
zebra danios (Danio rerio)
Dwarf pufferfish (Carinotetraodon travancoricus)
neon tetras (Paracheirodon innesi)
cardinal tetras (Paracheirodon axelrodi),
black neon tetras (Hyphessobrycon herbertaxelrodi
Endlers guppies (Poecilia wingei),
chili rasboras (Boraras brigittae),
harlequin rasboras (Rasbora heteromorpha)
pygmy corydoras (Corydoras pygmaeus),
the apple snail (Ampullariidae family
mystery snail (Pomacea bridgesii)
Fancy goldfish
Fancy goldfish need 20 gallons (75.7 L) of water per fish for the smaller varieties, with larger fish needing even more space.
Additionally, since goldfish are highly social fish, keeping only one per tank is not right. Most goldfish keepers recommend an additional 10 gallons (37.9 L) for every additional fish added.
However, given their heavy bioload, it’s a much better idea to aim for 20 gallons minimum per fish instead.
Too often, goldfish are won as a prize for entertainment. The truth is that a recipient rarely has a cycled aquarium waiting for their fish at home, and a 5-gallon tank is immediately available and affordable.
Due to the amounts of ammonia they create, their larger size, and their long lifespan, these fish barely last a week when kept in a 5-gallon setup.
Common goldfish (Carassius auratus)
You cannot keep common goldfish in 5-gallon setups for most of the same reasons. These fish do best when kept in ponds or extremely large aquarium systems with other goldfish.
The truth is that common goldfish can easily grow to be more than two feet (61.0 cm) long and create massive amounts of waste. More water volume allows for the physical size of the fish and dilutes the large amounts of ammonia entering the system.
Even when small, these fish create huge amounts of waste that will quickly shock a smaller system.
Danios & minnows
Freshwater fish like white cloud mountain minnows (Tanichthys albonubes) celestial pearl danios (Danio margaritatus), and zebra danios (Danio rerio) might seem small and manageable. But, these species of fish are highly active schooling fish that need a lot of space.
Though danios and minnows will mainly stay towards the top of the tank, they need plenty of space to move around. These hardy fish are some of the most active in the fishkeeping hobby and will quickly stress out in smaller tanks.
These fish should never be kept in anything less than a 20-gallon (75.7 L) long tank where horizontal aquarium floor space is greater than vertical.
Dwarf pufferfish (Carinotetraodon travancoricus)
Dwarf puffers are definitely size-appropriate for a 5-gallon tank, but unfortunately, hobbyists can’t meet their need for good water quality and enhancement in such a small tank.
Smaller tanks can easily cause water parameters to fluctuate, given how little water there is. More advanced hobbyists may achieve stability, but dwarf puffers will also need an engaging ecosystem with decorations to keep them from getting bored.
For one dwarf pufferfish, a 10-gallon tank (37.9 L) is recommended. If you want to have more than one puffer, at least 20 gallons (75.7 L) is needed.
Tetras
All tetras, including neon tetras (Paracheirodon innesi), cardinal tetras (Paracheirodon axelrodi), and black neon tetras (Hyphessobrycon herbertaxelrodi) cannot be kept in a small tank. Even though these fish are some of the smallest available, they are still active schooling fish that need room to swim.
Like the white cloud mountain minnow, tetras become stressed when kept in groups of less than eight individuals. If you want to see these fish at their best, set them up in a larger, more extended tank with plenty of companions.
Guppies
Guppies, including Endlers guppies (Poecilia wingei), are usually kept in really awful conditions in pet stores. This does not mean they can be shoved into a tiny nano tank forever.
These fish grow large. Not only do they get bigger than you think they will, but they’re prolific breeders. It doesn’t help that even if you buy all females, they may be pregnant right from the fish store or breeder and spawn in your aquarium.The small space of a 5-gallon tank also doesn’t allow the females to escape males constantly harassing them to breed.
Rasboras
Another active genus of fish, rasboras, needs larger tanks and greater numbers to thrive.
Again, it should come as no surprise that even the tiniest of rasboras, such as chili rasboras (Boraras brigittae), still do best in large groups and spaces; the ideal tank size for a school of chili rasboras is at least 10 gallons (37.9 L).
Larger species, like harlequin rasboras (Rasbora heteromorpha), need at least 20 gallons (75.7 L) to feel at home.
Corydoras spp.
Everyone wants to have corydoras in their aquariums! These small, active fish bring life and excitement to the bottom of the tank in a way that other species can’t.
However, all corydoras species, including one of the smallest species, pygmy corydoras (Corydoras pygmaeus), are too active and/or too large for a 5-gallon aquarium.
Pygmy corydoras are also sensitive to poor water quality, which can be challenging to control in smaller setups. Since 5-gallon aquariums tend to have less water chemistry stability, any slight change can spell disaster for the pygmies.
Although many people recommend getting just one corydoras as a “cleaner” fish for smaller tanks, don’t be fooled! Corydoras are schooling fish and need others of the same species.
Hobbyists should also note that corydoras can’t survive on algae alone!
Large snails
In particular, the apple snail (Ampullariidae family) and mystery snail (Pomacea bridgesii) grow too large and produce a lot of waste.
People always underestimate how much waste snails produce. However, if the tank is adequately filtered with live plants and no fish, large snail nano tank setups can be successful.
To make life a bit easier on yourself overall, aim for bigger setups for larger snails. In a small tank, stick to nerite snails that can’t breed in freshwater.
I hope this helps you out with your aquarium journey.