13 Great Fish for 3.5 Gallon Aquariums (& How Many of Each)

Have a 3 or 3.5 gallon fish tank laying around and want to know what you can put in it?  There aren’t many options, but you’re in luck.

The best fish for a 3.5 gallon tank are least killifish, endler’s livebearers, and betta.  These tanks are tiny, but you can keep a single betta or a pair of the others. Red Cherry Shrimp, Rili Shrimp, and Crystal Red Shrimp are better options, and you can keep 10 or more in a species only tank.

There are a few other options, however.  Let’s discuss them.

Note: If you have the option, I would suggest upgrading to a 5 gallon tank.  These will work a lot better in one of those, and it will make them easier to take care of.  They don’t take up a lot of space, and you can get one for $5 at PetCo’s dollar per gallon sale.

Least Killifish

Mosquito fish (Heterandria formosa), also known as the least killifish. | Source: Deposit Photos

The least killifish is not actually a killifish.  It’s actually a livebearer that stays absolutely tiny.  Because of this, they can live in a super small 3 gallon tank, even if it isn’t ideal for them.

Because they’re livebearers, they breed out of control if you let them.  This means you should only keep a pair of males in the tank together.  If you get females, even if you don’t have any males in your aquarium, they’re likely to already be pregnant and start producing offspring.

This will quickly overrun a 3 or 3.5 gallon tank, and you’ll end up with problems pretty quickly.

How Many? 2-3 males in a species-only tank.

Ghost Shrimp

Ghost Shrimp | (C) Tiny Underwater CC-BY-4.0

The first shrimp on the list is the ghost shrimp.  This shrimp earned its name by being almost entirely see-through.

In reality, ghost shrimp is a catch-all name for any translucent shrimp that isn’t otherwise kept in the aquarium hobby.  Sometimes, the shrimp aren’t even freshwater shrimp.  A lot of them are brackish shrimp that just happen to do okay in freshwater for a while.

Because of this, ghost shrimp can be difficust to take care of and even more difficult to breed unless you keep them in brackish conditions during breeding.

Ghost Shrimp | (C) Tiny Underwater CC-BY-4.0

Ghost shrimp often die early because they are wild caught and mistreated during transport.  Still, because they are the cheapest shrimp you can get, it might be worth trying them out if you’re unsure whether you want to keep shrimp or not.

How many? 10 per gallon is a good rule of thumb, but it may be less if you have a tall/skinny tank with not much floor space.  I’d recommend only adding 10 at a time.

Endler’s Livebearers

Endler’s Livebearer Male | Source: Deposit Photos

Endlers livebearers are a colorful livebearing fish that is a close relative of the guppy.  They look great, and because they are so small, they can be put in a 3.5 gallon aquarium.

Like the least killifish, you only want to keep a pair or trio, and you want to keep all males.

Endlers are relatively easy to take care of, though since this is a tank at the very smallest size they can be kept in, you’re likely to need to pay more attention to water quality and maintain a regular water change schedule to keep them happy.

How Many? 2-3 males.  Avoid keeping females to prevent them from reproducing and overpopulating the tank.

Red Cherry Shrimp

Red Cherry Shrimp | Source: Deposit Photos

Red cherry shrimp are one of the most popular shrimp in the hobby.  They are one of the easier varieties to take care of, and (unlike the ghost shrimp) they will commonly breed in freshwater aquariums.

RCS come in a variety of grades, with the better grades indicating better colors.  At the low end you have regular RCS that are a mostly translucent reddish color, and at the high end you have bloody mary RCS that are a deep red.  They also come in a Kanoko variant that has black patches on its back in addition to the dark red color.  (To the best of my understanding.)

Male cherry shrimp in a planted aquarium | Source: Deposit Photos

Like other shrimp, RCS can be kept nicely in small aquariums due to their small size and negligable bioload.  You’ll still want to do regular water changes to keep the water quality good, but you’ll have an easier time with this pick than you would with the fish on this list.

How Many? 10 per gallon, so up to 30-35 in a short, wide 3.5 gallon tank.  Start with 6-10 of them, since they will breed on their own if they’re happy.


My Betta in Front of My Ottos | Source: Tiny Underwater | License: CC-BY-4.0

This is probably the #1 fish you were expecting, and it’s the most commonly bought fish for this size of tank.  I’m sure I don’t have to tell you about the nearly endless variety of colors and fin shapes you can get this fish in.

I actually recommend a 5 gallon or bigger for betta, especially since they’re not much more expensive or larger in footprint than a 3 or 3.5 gallon but come with 43% more water for your betta to swim in.

Betta fish, siamese fighting fish, betta splendens | Source: Deposit Photos

Betta are extremely easy to take care of, and food for them is widely available.  If you want to get into keeping a fish that is beautiful, packed with personality, and will lead to you always wanting just one more aquarium, this is the one to go for.

How Many? You should only ever keep 1 male betta in an undivided tank, and this tank is too small for female or plakat bettas to be kept in.  Avoid dividers for this size tank.

Crystal Red Shrimp

Crystal Red Shrimp Eating (Cardinia cantonensis) | Source: Deposit Photos

Crystal Red Shrimp are a type of Cardinia shrimp that come in beautiful red and white striped patterns.  In my opinion, they look better than red cherry shrimp.

CRS have more specific water requirements than RCS and are in general said to be more difficult to take care of.  Because of this, if you think you might want to take care of them, you should do your research before hand to make sure you can provide them the right conditions.

Of course, this means you need to be extra diligent about water changes. 3.5 gallons is on the small side for the Crystal Red Shrimp, so water quality is not something you can slack on.

Cardinia Red Bee Shrimp | Source: Deposit Photos

Like RCS, these will breed in your aquarium if you keep them happy.

How Many? With Cardinia shrimp, you should go with a rule of thumb of 3 per gallon, rather than the 10 for the cardinia shrimp.  Keep no more than 9-10 in a tank of this size.

Mystery Snail

Mystery Snail | (C) Tiny Underwater | License: CC-BY-4.0

Mystery snails are another option for a 3 or 3.5 gallon aquarium.  Some sources say 5 gallons is the smallest size aquarium they can be kept in, but for a 1 snail aquarium, it should be fine.

Mystery snails come in a few different shell and skin colors, including brown, white, and gold shells and pink, blue, or brown skin.  They can actually look quite nice if you’re into snails, and they’re my favorite variety of large snail.

If you’re keeping a snail-only aquarium, I recommend planting it out heavily and adding some flow for more visual interest.  Your snail will get pretty big and will look like a titan while stalking around the aquarium.

In my larger aquariums, I seldom needed to feed my mystery snail, but in one this size you’ll want to regularly feed algae wafers.  Snails can be prone to die-offs if the water quality gets too high in certain elements that are toxic to them, so you’ll want to change the water and avoid using anything with copper in it.

How Many? Only 1 mystery snail is appropriate in a tank less than 5 gallons.

Black Bee Shrimp

Black bee dwarf shrimp, Cardinia | Source: Deposit Photos

Black Bee Shrimp are related to the Crystal Red Shrimp, but instead of coming in a red and white stripe pattern, they come in a black and white pattern.

They look good, but like the CRS they can be more difficult to take care of than Neocardinia shrimp like the Red Cherry Shrimp.

Black bee shrimp need slightly acidic water in the range of 6.2-6.8 PH, and they need warm water in the range of 72-78F. Stability is more important than the specific value, though, so it’s important to keep the water parameters from fluctuating too wildly.

How Many? 3 per gallon, so no more than 9-10 in a 3.5 gallon tank.

Rili Shrimp

Photo 192825859 / Rili Shrimp © Dm Stock Production | Dreamstime.com

Rili shrimp are a type of Neocardinia shrimp, coming from the same species as the Red Cherry Shrimp, just bred for their interesting banded translucent and red, blue, green, orange, or black colors.

Like the RCS, they are going to be one of the easier types of shrimp to care for, and as long as you keep the water parameters in the range of 6.8-7.5 PH and somewhere between 68-86F they should be quite happy.

Photo 204816818 / Rili Shrimp © Dm Stock Production | Dreamstime.com

Stability here is more important than the specific value, much like the Black Bee Shrimp.  If they’re happy, they’ll start breeding.

How Many? Like other Neocardinia species, 10 per gallon is fine.  They will breed if you keep them happy, so start with an initial colony of 10 and let them increase on their own.

Nerite Snail

spotted nerite snail (Neritina natalensis) | ID 166024520 © Joan Carles Juarez | Dreamstime.com

Nerite snails are another large species of snail that should be able to be kept in a 3.5 gallon tank.

They come in a wide variety of colors and patterns, some with horns and others with smooth shells.  They are excellent little algae eaters, and like the mystery snail they will need to be specifically fed in a tank of this size.

Unlike mystery snails, they are escape artists, and you need a tight fitting lid with no holes to keep them from packing their bags and moving as soon as they decide that conditions in the aquarium aren’t to their liking.

Photo 143352821 / Horned Nerite Snail © Ezthaiphoto | Dreamstime.com

They will also leave small, sesame seed sized eggs all around the aquarium.  (These won’t hatch unless you make it a brackish tank.)

How Many? Only 1 large snail for a tank this size.

Blue Velvet Shrimp

Blue Velvet Shrimp, Neocardinia | Photo 92226988 © Serhii Shcherbyna | Dreamstime.com

Blue velvet shrimp are another Neocardinia shrimp of the same species as Red Cherries or Rili Shrimp.  They are my personal favorite, since unlike the others they come in a deep blue color that really makes them stand out in an aquarium.

Like the others, they are easy to care for.

One thing to note is that, if you decide to breed them, you need to remove the less colorful (more translucent or otherwise ugly) shrimp to another tank to keep them from breeding with your better quality shrimp.  Whether you do this or not will dictate whether your shrimp stay super colorful and nice looking or whether they turn into a bland, dull batch of low grade shrimp colors.

How Many? Like other neocardinia species, 10 per gallon.

Snowball Shrimp

Snowball Shrimp AKA White Pearl Shrimp | Photo 109345767 © Danolsen | Dreamstime.com

Snowball shrimp are another Neocardinia shrimp, but unlike the ones we’ve covered so far, these are Neocardinia zhangiajiensis instead of Neocardinia Davidi.  This means they are a completely different species.

Luckily, they are easy to care for, and in terms of care requirements, you should be able to start using any of the guides for taking care of Red Cherries for these shrimp.

The largest difference you’re likely to notice is what colors they are.

They’re also supposed to be easy to breed, so you should only ever need to buy a small starter colony.  (And maybe some from time to time to diversify your genetic lines if you really get into shrimp.)

How Many? 10 per gallon, like other neocardinia shrimp.  30-35 total for a tank this size.

Bumblebee Shrimp

If you’re looking for a bit of a challenge, freshwater bumblebee shrimp are an interesting and less kept variety of Cardinia shrimp.  They are a different species from the Crystal Reds and Black Bee Shrimp, and they have slightly different care requirements.

A larger tank would probably be best here, but if you’re really invested in a tank this small, they should still do fine.

Unfortunately, there is not a huge amount of information about caring for this type of shrimp, so this should not be the first type of shrimp you take care of.  If you’ve already had experience caring for other cardinias, however, this may be a good one to try next.

How Many? I don’t have a specific answer for this species, so it’s probably best to go with the general 3 per gallon rule of thumb for Cardinia shrimp and start with a small colony.