How many fish should be in a 20-gallon tank?

When setting up a new aquarium, one of the most important considerations is stocking levels – how many fish can live comfortably in the tank. This largely depends on the size of the tank and the types of fish being kept. For a standard 20-gallon aquarium, here are some guidelines on recommended stocking levels.

General Stocking Guidelines

As a general rule of thumb, it is recommended to have 1 inch of adult fish per 1 gallon of water in the tank. A 20-gallon tank would therefore support about 20 inches of fully grown fish. However, this does not mean you can add 20 small fish to the tank. The inch-per-gallon rule only applies to the maximum adult size of the fish, not their current juvenile size.

For example, a single 5-inch goldfish would already take up 1/4 of the stocking capacity of a 20-gallon tank. Adding 19 more goldfish would greatly overload the tank. Similarly, a school of 20 neon tetras, which can reach 1.5 inches at maturity, would equate to 30 inches of fish – exceeding the recommended 20-inch limit.

It’s also important to note that the inch-per-gallon rule is simply a guideline, not a hard and fast rule. Some active, messy fish require more space than their size would suggest. Territorial species may need to be kept singly or only with certain tank mates. And certain fish species thrive better in larger groups, requiring more room.

Recommended Stocking Levels by Fish Type

Here are some general stocking recommendations for a 20-gallon aquarium based on different types of fish:

Small Tetras

Fish like neon tetras, ember tetras, and glowlight tetras that reach 1-1.5 inches can be kept in schools of 8-12 individuals. Avoid overcrowding by sticking to one small tetra species.

Medium Tetras

Slightly larger tetras like black skirt tetras, rummynose tetras, and cardinal tetras that get to around 2 inches are best kept in groups of 6-8 fish. Only keep one tetra species and watch aggression levels.

Large Tetras

Tetras reaching 3 inches or larger, like Congo tetras, should be kept in schools of 4-6 fish. Make sure to choose peaceful varieties that won’t terrorize smaller tank mates.


Active danios like zebra danios and pearl danios can be kept in a school of 6-10 fish in a 20-gallon tank. Avoid fin-nipping varieties like tiger barbs that may harass other fish.


Boisterous rainbowfish do best in groups of 5-6 individuals. They require ample swimming space so are best for longer tanks.


A 20-gallon tank has room for 1-2 small gouramis like honey gouramis or sparkling gouramis. Only keep 1 male to avoid aggression and fighting.


This size tank can house a pair of angel fish. Make sure tank mates are not small enough to be eaten.

Dwarf Cichlids

A single dwarf cichlid like a ram can work well as a centerpiece fish. Avoid mixing multiple cichlid species in such a small space.


One small pleco like a bristlenose or rubberlip pleco is suitable for algae control. Avoid common plecos that get too large.

Corydoras Catfish

Keep 5-6 corydoras catfish to see their best schooling behavior and activity. Provide fine sand for them to sift through.


A trio of mollies will add interesting colors and activity. Avoid chasing by keeping 2 females per male.


A 20-gallon tank can support 4-6 lively platies. Like mollies, aim for at least 2 females per male.


Keep 3-4 swordtails in a mix of males and females. The larger, more active males may harass females.


Avoid overcrowding by keeping just 1 male and 2-3 female fancy guppies. They will breed prolifically.

Dwarf Shrimp

20 gallons provides room for a large colony of small shrimp like cherry shrimp or ghost shrimp. Provide plenty of hiding spots.


Several nerite snails, mystery snails, or other small varieties can help clean up algae without overwhelming the tank.

Water Parameters

In addition to stocking levels, water quality influences how many fish can be successfully kept. Ammonia and nitrite levels should be 0 ppm, while nitrates should be below 40 ppm in a healthy cycled tank.

Perform regular partial water changes of 25-30% weekly to remove nitrates and replenish minerals. Clean the filter monthly to remove gunk and ensure optimal water flow.

Use a liquid test kit, not test strips, to check pH, hardness, and other parameters match the needs of the fish species. For community tanks, aim for a pH around 7.0-7.5 and hardness around 5-12 dGH.

Tank Setup

The tank setup also affects carrying capacity. Make sure the 20-gallon tank is properly cycled before adding fish. Have efficient filtration that can handle the fish bioload. Heavily stocked tanks may need stronger filters or extra powerheads for better water movement.

Provide plenty of hiding spots with rocks, driftwood, and plants. Floating plants help soak up excess nutrients and provide shade. The more territory and visual breaks, the less aggressive fish will be.

Arrange decor in a way that utilizes all levels of the tank. Bottom dwellers like cory cats need open substrate space to forage. Middle swimmers like tetras prefer some open water. Surface fish enjoy floating plants for cover overhead.

Watching Water Quality and Behavior

When first setting up the tank, add fish slowly over several weeks. Start with just a few hardy species like danios or platies. Monitor ammonia and nitrite levels to ensure the tank can handle the bioload as fish are added.

Observe fish behavior to spot signs of aggression or stress from overcrowding. Consider removing territorial or very active fish that seem to disrupt tank mates.

Overfeeding should be avoided as this fouls the water. Feed only as much as the fish can consume within 2-3 minutes 1-2 times per day. Remove any uneaten food to avoid excess nutrients.

With close attention to water parameters and fish behavior, a 20-gallon tank can comfortably house a vibrant community of small tropical fish species!

Tank Size Recommendations for Common Aquarium Fish

Fish Type Minimum Tank Size
Neon Tetra 10 gallons
Betta Fish 5 gallons
Guppy 10 gallons
Platy 10 gallons
Molly 10 gallons
Zebra Danio 10 gallons
White Cloud Mountain Minnow 10 gallons
Corydoras Catfish 10 gallons
Angelfish 20 gallons
Discus 40 gallons
Oscar 55 gallons
Common Pleco 55 gallons
Goldfish 20 gallons
Koi 55 gallons


When stocking a 20-gallon aquarium, focus on selecting smaller species that exist peacefully in groups. Suitable options include small tetras, rasboras, danios, dwarf cichlids, gouramis, mollies, platies, cory cats, and invertebrates like shrimp and snails. Avoid large, messy, or aggressive fish that require much more room.

Aim to house no more than 20 inches of adult fish, with most schools containing 4-8 individuals. Go slowly when adding fish and monitor water parameters closely. Make sure filtration is robust and the tank is structured to create territories and line-of-sight breaks. With close attention to setup and monitoring, a 20-gallon tank can be a thriving community aquarium.

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