How Many Fish Can Safely Live in a 20-gallon Tank? 6 Things to Know

You’re a fairly new fish owner dreaming of greater tanks after starting with a 5-gallon or 10-gallon tank, or maybe even just a goldfish or betta fish bowl. You’re ready to upgrade to that 20-gallon life. Or maybe you’re brand-new to fish parenting, but you’re ready to dive into the deep (or more capacious) end right away!

Either way, you probably have a lot of questions: how do I transfer my existing fish into the larger tank, or how can I grow my fish family without a lot of inter-species strife? How much and how often do I need to change the water on a 20-gallon tank?

In this article, we’ll cover everything you could want to know. We’ll start with how many fish can safely live in a 20-gallon tank, but then we’ll submerge ourselves even further in the topic. Here are 6 things to know.

How Many Fish Can Safely Live in a 20-gallon Tank? It All Depends

Let’s begin by admitting that the question we posed is overly broad, and explain why. In a word, fish are all different, just like Dr. Seuss tried to warn us. Some species of fish are highly aggressive — if you put them in a tank of any size with a neighbor, there will be drama. So in that case, the answer to the question of how many fish can safely live in a 20-gallon tank would be, um, one. Some fish will only do well in a single-species tank, such as tiger barbs or piranhas.

Other factors you need to consider when filling your 20-gallon tank with fish are:

• Their size (not only on purchase but when they grow to adulthood).
• The climate they prefer.
• Whether they are nocturnal or diurnal.
• The pH balance of the water in which they can live. Whether they are saltwater or freshwater fish.
• Whether they are of a species that requires more or less room to roam.
• How many physical structures and decor you plan to add to the tank (and how much of this stuff your fish need).

But as a very broad general rule, a 20-gallon tank can probably support up to 20 fish, with up to 10 other species such as invertebrates, snails and other bottom-feeders. Read on to learn the many provisos and exceptions to that rule…

Bioload: The Most Important Factor For Stocking a 20-gallon Tank

The two most important factors that will determine how many fish can safely live together in your 20-gallon tank are the size of the fish and the fish’s bioload.

In short, bioload is the reason your fish tank has, and needs, a filter. Brief chemistry lesson incoming: there are many natural processes your fish go through that create nitrogen. These processes include neglected food, decomposed fish that die off, and the main one, fish poo. Your fish tank’s filter exists to remove the build-up of nitrogen in your fish’s water. If it were allowed to stagnate, it would eventually poison your tank’s inhabitants.

Why doesn’t this happen in nature? There are no nitrogen filters in ponds, after all. But the explanation comes from your fish tank’s smallness of size. A pond or other water body has thousands of times more space per fish, and it’s out in nature, where nitrogen is naturally filtered by plant life, dilution, evaporation and motion, or even utilized by non-fish species. It’s a human-made wonder, really, being able to keep swimmy creatures happy in a space far smaller than they’d have in the wild.

But if you have too many fish that are too large, they will produce more nitrogen bioload than your filter can handle. This produces lots of bad effects: the filter will likely clog; the water will become dirty; nitrate and ammonia will build up and become toxic.

If you have live plants in the tank, they can absorb some of the nitrogen load. But the plants can’t do it alone. So you’ll want to be sure your total fish bioload is well within the range of what your tank’s filter can handle.

Species of Fish That Thrive In a 20-gallon Community Tank

Now we know that the number of fish that can safely live in a 20-gallon tank is actually a range of numbers. Let’s look at several different species of pet fish that can coexist well in a 20-gallon tank.

Honey gouramis are sometimes called centerpiece fish. Growing up to 3 inches long, they’ll have enough room to live in a 20-gallon tank (so long as you don’t overpopulate it). Unaggressive and flexible, you can add just one honey gourami to your tank, pair them up, or add several. There are lots of other species honey gouramis can learn to live harmoniously with.

Neon, cardinal, green and ember tetras, respectively sporting iconic racing stripes of various bright hues, like to live in schools of at least a half-dozen. So long as they have a school to call their own, they can share your 20-gallon tank with numerous other species.

Chilli rasboras are quite wee, growing to less than an inch. In spite of being so diminutive, they’re quite social and bold with other fish. They should also be kept in a school of 6 or more.

Celestial pearl danios are small, delicate, and shy fish often described as looking like a miniaturized version of brook trout. With their striped fins and spotted bodies, it’s easy to see why. They also have a delightful coloration that shimmers and shifts under different light conditions.

Pygmy corydoras grow to about an inch in length, and, unlike other small fish, like the mid-layer of your 20-gallon tank, adding visual variety. They’re sometimes schooling fish who occasionally strike out as individuals. A minimum of seven is recommended for the greatest thriving.

Breeding Fish In a 20-gallon Community Tank

Some people may like to bring live bearers — fish able to reproduce — into their 20-gallon tanks. The first thing to keep top of mind if you house breeding fish in your aquarium is that the number, mass, and bioload of your tank will change with the new additions. If you don’t intend to keep all the new fish, you’ll need a plan for rehoming them, just as if your cat were having kittens.

You’ll also want to be aware of certain diseases live bearers can carry or transmit, and know how to spot the signs. Finally, you’ll want to keep a ratio of 3 females to every male in the tank to prevent overmating. Small live bearers recommended for 20-gallon tanks include guppies, mollies, and platys.

Species of Fish That Don’t Do Well In a 20-gallon Tank

Basically, some species are just too large to thrive in a 20-gallon tank. These include fish that are obviously too large even in youth, such as koi, arawana, alligator gar, and bichir. But what you really want to watch out for are the kind of fishies that are very small when you purchase them, but then grow too large for 20 gallons. Fish that will outgrow your 20-gallon tank in a hurry include angel fish and goldfish.

20-gallon Tank Aquarium Ideas

Time to synthesize all this and look at the whole aquarium at once. Here are a few different ways you can go with adding fish (and friends, and everything else they’ll need) to your 20-gallon tank.

(Mostly) Single-Species Tank

Let’s say you’re new to the aquarium game or you just want to keep things simple, single-species, and aesthetically attractive. Try this simple approach: fill the side of your aquarium closer to the wall with a variety of plant life — good ones include dwarf aquarium lily, java fern, and Vallisneria.

Populate this waterscape with a single school of neon tetras; they’re bright, beautiful, and like to school in the middle of the tank. 12 to 20 tetras can live comfortably together in your 20-gallon tank, along with a few bottom dwellers. Red cherry shrimp stand out nicely against the green foliage.

There are species you can add to your tank that will actually help keep it clean, by eating certain types of refuse and algae. These include nerite snails and kuhli loaches.

Live Bearer Tank

If you’re interested in fish that breed, we’ve already mentioned guppies and platys. Many people also enjoy breeding bristlenose plecos (also called bushynose plecos). They come in various colors and styles — albino, brown, calico, super red, and long-finned varieties — and have been specifically developed to be easy to breed.

You’ll need to provide your male pleco a cave, as it’s part of the pleco mating process. The male invites the female into his cave, where she lays eggs which he fertilizes. The male then fans the eggs with his fins, encouraging them to hatch.

A 20-gallon tank works great for a pleco family. Start with just mom and dad, and soon they’ll have a whole brood swimming about happily. If the parent plecos mated in a preexisting community aquarium, you can move them to their own 20-gallon home within two days of the hatch. Don’t forget to bring the cave!

You’ll definitely want some nice greenery in your pleco tank. Driftwood, java fern, and Anubias make a great nursery for baby plecos, and they can snack on the wood to develop their digestive systems.

Once the young ones are 2 inches long, you can either redistribute them into your other aquariums (they’re a great addition to a tank in need of algae control) or sell them to a pet store. You can then repeat the breeding process as many times as you like.

Invertebrate Tank

It’s not all about fish! Perhaps you’d prefer to build your 20-gallon aquarium around something a touch more bizarre and exotic. There are a number of fascinating shrimp you can keep in a tank of this size: bamboo, wood, and vampire shrimp. These shrimp are known as filter-feeding animals, who survive by using their weirdly “feathery” hands to catch tiny edible particles floating in the water. Because of that, these animals have different filter requirements. In fact, if you over-filter their water, they’ll have too little to eat! You’ll want a gentler sponge filter or air stone.

Flora here are essential, too. Your shrimp need lots of sturdy plant structures to climb on as part of their particle-hunting. Remember your pet is the filter with these invertebrates: when you feed them (with raw powder food or the specialty shrimp food your pet store recommends), expect the water to be a bit cloudy, with food particles visibly floating around.

These shrimp grow up to six inches. For a 20-gallon tank, the ideal would be one or two bamboo shrimp and one vampire shrimp. They live well alongside nanofish like the danios and rasboras we mentioned earlier and will leave enough tasty leftovers to support the much smaller bottom feeders, such as snails, amano shrimp, and cherry shrimp.

Cool Water Tank

This tank is built around the swift, peppy Danio. It comes in many colorways and variations: long-finned, leopard, zebra, and glowing. It likes a larger school of 12 to 15, which is a perfect number for your 20-gallon tank. They love to zoom around in swift unison and form quite the frenzy at feeding time. All in all, they pack a lot of entertainment and visual attraction into a small package.

The added benefit of Danio fish is, that you don’t have to heat the tank! That is, as long as the room you keep them in doesn’t dip below 62 degrees Fahrenheit. They’ll swim zippily to all corners of your aquarium, but they can also coexist with other species, so long as those species also like things on the cooler side. A 20-gallon tank with a school of danios will also support up to five salt and pepper corydoras, who can live on the food the danios drop. As for invertebrates that don’t need a warm environment, you could introduce amano shrimp and a variety of snails: Japanese trapdoor snails, nerite snails, and Malaysian trumpet snails all work. (You will need to add food specifically for them, rich in calcium, to the tank.)

How Do I Know If There Are Too Many Fish In My 20-gallon Tank?

If you’re regularly monitoring the health of your fish, you’ll be able to see potential signs that you’re overstocked and your habitat is retaining too much nitrogen.

Symptoms to look out for include:

• Fish behaving unusually lethargic, moving less and more slowly.
• Fish hanging around near the surface of the water.
• Fish should typically visit water outlets, where the filter is creating fresh water. If they’ve stopped doing so, this is a symptom of overstock.
• Fish’s gills moving unusually quickly.
• Fish’s gills turning brown.
• If the water in your tank looks dirty, cloudy, or as if it has too many particles floating in it, this can be a visual sign of an overloaded filter. (Make exceptions to this rule if you keep filter feeder animals in your 20-gallon tank.)

If you notice any of these symptoms in your 20-gallon tank’s denizens, you’ll need to move some of them to a new habitat. You’ll also want to cycle out all the water in your tank and possibly take other steps to clean it and its filter, ensuring that your pets can return to a healthy aquarium as soon as possible.

The photo featured at the top of this post is © Varvara Voinarovska/Shutterstock.com