How Many Neon Tetras Can I Keep in a 20-Gallon Tank?

One of the hardest parts of keeping fish is the fact that tanks can only accommodate so many before they become overstocked. In fact, a tank that has relatively few fish may look empty but be at capacity, or even over it. This is especially true if you have a relatively small tank with a school of fish in it. Schooling fish are great fun to watch, but when you’ve overstocked your tank, your tank won’t be able to accommodate everything. This is because your tank itself is one big ecosystem that allows it to function properly. Safely, you can keep between 10 and 20 neon tetras in a 20-gallon tank, so long as there are no other fish with them.

(Check out our guide on caring for neon tetras here!)

Stocking Density of a Fish Tank

When we talk about keeping fish in a tank, the amount that you can keep is the stocking density. The number and choice of fish that you choose to keep make a difference to the health and welfare of your tank. While you may think that having three big fish is no different than three small fish, this is not the case.

Different sized fish require different amounts of water dedicated to them. Being able to stock your aquarium properly requires you to know the volume of water you keep and the filtration capacity. However, there are also shortcuts that you can take to estimate the right numbers for your tank.

Inches of Fish per Gallon

Many people cite the 1 inch of fish per gallon of water rule when trying to estimate how many fish they can keep in their tank. While this is a good way to get the general estimation of fish, it is not very exact, nor does it account for how demanding some fish are compared to others. The 1 inch per fish rule is also pushing your stock pretty far as well– many people prefer to use the more conservative estimate of 2 inches per fish.

Filtration Capacity

You also have to consider the filtration capacity when considering the density of your tank as well. The filter is essential to keeping toxins in your tank to a minimum. A filter should provide plenty of space for bacteria to gather in your tank to help break down toxins. It then pulls the water in, filtering out large debris, waste, and the like while also passing the water through the bed of bacteria. The bacteria can then break down the chemical waste in the water and keep it clean and healthy for the fish and plants in your tank.

If your filter is too small for your tank volume or if you have a heavily stocked tank, it’s usually a good idea to err on the side of providing extra filtration to keep your water circulating. Without the proper filtration, your tank will likely build up toxins and could also cause a spike in algae growth. This will stress out your fish and make it harder to keep them alive. The more fish you have, the more filtration capacity you will need.

Why Does Stocking Density Affect Water Parameters?

When you stock a tank, your water parameters will naturally change. This is because as you add fish in, you add additional bioload that has to be processed. This processing happens thanks to the bacteria that live in your filter. As you add fish, your bacteria load has to grow to accommodate the rising numbers of toxins.

Because tanks with more fish and a higher stocking density will have more bacteria, the water parameters will be more likely to vary. If your tank isn’t cycled, you could see massive spikes in numbers that could be a problem. By allowing your tank to cycle properly and become mature, you will have a cycle that sustains itself.

Your fish produce ammonia as they produce waste. This ammonia is toxic to your fish in high levels. That ammonia has to be broken down by bacteria, and as it is, it is turned into nitrite. However, nitrite is even more toxic to your fish than ammonia. That nitrite then has to be broken down by bacteria as well, and they turn it into nitrate. Nitrate is generally harmless to fish in small quantities and will be used up by plants in your tank if you have them. This cycle continues constantly in your tank. To prevent excessive buildup of nitrate, you do a small water change each week.

What if I Overstock?

If you overstock your tank, you run the risk of the bacteria not being able to keep up with the waste produced by the fish. The guidelines for stocking tanks are designed to prevent this from happening because fish produce waste more or less proportionately to their general size. Larger fish will produce more waste and smaller fish will produce less. The waste will build up to higher levels than the bacteria will be able to process and as a result, you could unintentionally stress or kill your fish as ammonia or nitrite levels increase.

Some skilled aquarists are able to overstock their tanks somewhat, but this requires extensive knowledge of aquatics, extra filtration, oxygenation, balancing the tank’s parameters, a heavily planted tank, and regular water changes. It’s generally not a recommended process for beginners and can quickly go wrong if you don’t know what you are doing.

What if I Understock?

Fortunately, understocking your tank isn’t a big concern. There are no real risks to understocking, so long as you also have enough fish for the species you have. Neon tetras, for example, require schools and should never be kept in groups of less than 5.

How Many Neon Tetras Can I Fit in a 20 Gallon Tank?

Using the information provided, you should be able to keep between 10 and 15 neon tetras in your tank without much problem. Neons tend to grow to between 1 and 1.75 inches in length. If you were to use the 1 gallons per fish rule, you would be able to comfortably fit 13-20 fully-grown neons in a 20-gallon tank. However, using the 2 gallons per fish rule, you would get a number of 7 at 1.5 inches each.

If your tank is relatively empty and is new without having time to cycle, you want to put as few fish in the tank as possible to give it time to grow and flourish. If you have a 20-gallon long tank as opposed to a tall tank, with plenty of filtration and plants, you could probably fit 15 to 20 in your tank if you kept up with water changes and didn’t stock any other fish in the tank.

Remember that these are general guidelines. A tank with just 7 tetras may run into problems if it has not cycled yet while a well-established tank with 20 may thrive. It depends on more than just the tank size. It is always a good idea to start conservatively and work up to the numbers that you would like to see. After all, keeping aquariums isn’t about racing to the finish line– it’s about providing a healthy environment for your fish to thrive while enjoying the process.

What if I Want Other Fish?

If you want more than just neon tetras in your tank, then you have to accommodate them. You will need to only add in as many fish as can comfortably fit, using the same rules. If you had, for example, a betta and two African dwarf frogs in the tank already, you would already be using 2-4 gallons for the betta and an additional 5 gallons for the frogs. You could only stock between 5 and 10 neons into the tank comfortably while considering the size and bioloads of the fish you already had. Alternatively, you can also get a larger tank if you want more than what your 20-gallon will allow.

If you are dead-set on having as many neons as possible in your tank, but still want to see other creatures as well, there are other ways that you can add some diversity to your tank as well without worrying about the bioload.

(We have a great guide on compatible tank mates for neon tetras – check it out here!)


While not fish, shrimp can create a fun addition to your aquarium and your neons aren’t likely to eat them. Shrimp come in a wide variety of shapes, colors, and sizes, so you can find something that works for you. Some popular beginner-friendly shrimp include:

  • Amano shrimp
  • Cherry shrimp
  • Ghost shrimp
  • Bamboo shrimp

By adding shrimp, you will add some cleanup crew to your tank. They will happily eat any food that falls to the bottom, as well as any algae they can reach. You can add dozens of shrimp without any real measurable difference in bioload, allowing you to get that variety you want in your tank without sacrificing the number of neon tetras you can keep.

Amano shrimp is a popular choice with beginners


Like shrimp, snails usually don’t add much to the bioload while helping to keep your tank clean. However, many different species will reproduce quickly and asexually, so they can quickly populate your tank if you aren’t careful. Mystery snails are a common addition to aquariums because they require a mate in order to reproduce and their eggs are easily spotted above the waterline before they can breed.

Snails can help keep your tank clean