How many fish can a 20-gallon tank have?

When setting up a new fish tank, one of the most important considerations is stocking it properly. Overcrowding your tank with too many fish can lead to poor water quality, disease outbreaks, and stressed, unhealthy fish. So how do you determine the right amount of fish for your tank size? There are a few key factors to take into account.

1. Know Your Tank Size

The size of your tank, measured in gallons, is the starting point for stocking calculations. A standard 20 gallon tank has dimensions of 24″ x 12″ x 16″ and holds approximately 20-22 gallons when filled. Knowing the exact tank volume will allow you to estimate the bio load (waste output) to ensure you don’t exceed the tank’s capacity.

2. Select Appropriate Fish Species and Sizes

Some species of fish reach relatively large adult sizes and require more tank space than smaller species. For example, common plecos can grow over 12 inches long and need a minimum of 30 gallons, so they are not suitable for a 20 gallon tank. similarly, goldfish can reach 8+ inches and prefer at least 30 gallons. Stick to smaller species of tetra, rasbora, danio, guppies, and dwarf cichlids under 2-3 inches adult size for a 20 gallon tank. Avoid overstocking with too many large fish.

3. Factor in Fish Waste Output

Fish produce waste in the form of ammonia, which gets converted to nitrite and then nitrate by beneficial bacteria in the tank. A heavier bio load means more waste accumulation, which must be removed through water changes and filtration. Smaller fish produce less waste than larger tankmates. A general rule is 1 inch of adult fish per gallon, adjusted based on fish species and filtration capacity. For a 20 gallon tank, aim for no more than 20 inches of relatively low waste fish.

4. Account for Tank Behaviors and Needs

Behavioral traits and environmental preferences also affect stocking limits. Territorial fish like bettas may fight if crowded. Schooling fish require groups of 6-10 of their own species. Scaleless fish like plecos need lots of hiding spots. Reef setups need strict water parameters. Make sure to research and accommodate the behaviors, social structures, and care needs of each fish species when planning compatible tankmates.

5. Provide Adequate Filtration

Filtration system capacity greatly impacts the stocking limit. Filters remove fish waste and debecompose ammonia into less harmful compounds. The more fish you add, the more waste produced, so filtration must be robust. Canister filters around 3-4 times the tank’s volume per hour are ideal for heavy stocking. Supplement with powerheads, undergravel filters, and regular water changes. Proper filtration is key to preventing toxic ammonia and nitrite spikes.

6. Allow Plenty of Open Swimming Space

Fish need adequate surface area and swimming lanes for natural movement and schooling behavior. Overcrowding causes stress, aggression, poor growth, and injury. A good rule is to leave 30-50% of the water volume open for fish to comfortably navigate. Arrange decor and plants to create open areas through the central water column from front to back glass. Tall plants can help define additional territories.

7. Start with a Minimum Number of Fish

It’s safest to slowly build up fish numbers over several weeks, allowing time in between additions for the biological filter to adjust. Only add a few hardy fish at first while monitoring ammonia and nitrite. Once the tank is cycled and water parameters are stable with the initial fish, you can steadily introduce more tankmates. Starting with too many fish at once risks an ammonia spike before bacteria colonies establish.

8. Quarantine New Fish Before Adding to Tank

Setting up a separate quarantine tank allows you to monitor new fish for signs of disease before adding them to your display tank. Quarantining helps prevent the introduction of contagious illnesses like ich and velvet. Treat or observe fish in quarantine for 2-4 weeks before moving to main tank. This protects existing fish and avoids disrupting the tank’s cycle if medications are needed.

9. Provide Proper Tank Maintenance

While a tank may seem suitable when perfectly clean, as waste accumulates it becomes less hospitable. Overfeeding and lack of water changes can degrade conditions over time in an overstocked tank. Stick to a strict regime of partial water changes, gravel vacuuming, filter backflushing, and testing for ammonia, nitrite, nitrate, and pH. More fish means more frequent tank maintenance is required to prevent problems.

10. Allow Room for Fish to Grow to Full Size

Many aquarists mistakenly choose fish based on purchase size, not adult dimensions. But most fish will rapidly outgrow their juvenile size within weeks or months. Make sure your planned stocking allows space for fish to reach their maximum growth. Check species profiles and don’t overload a tank with too many fish expected to enlarge to big sizes at maturity.

11. Some General 20 Gallon Stocking Recommendations

Here are some examples of suitable stocking options for a 20 gallon freshwater tank:

  • 1 dwarf gourami + 15 neon tetras
  • 1 betta + 15 harlequin rasboras
  • 12 cardinal tetras + 6 kuhli loaches
  • 1 dwarf cichlid pair + 8 cory catfish + 10 guppies
  • 1 honey gourami + 8 pygmy corydoras + 12 chili rasboras

For a 20 gallon planted tank, you could do:

  • 1 Betta + 6 pygmy corydoras + 10 ember tetras
  • 1 dwarf gourami + 6 panda corydoras + 8 glowlight tetras
  • 1 honey gourami + 10 cherry barbs + 5 otocinclus

A 20 gallon community tank could comprise:

  • 10 neon tetras + 6 panda corydoras + 2 guppies
  • 8 black skirt tetras + 6 habrosus corydoras + 5 platies
  • 10 bloodfin tetras + 5 pygmy corydoras + 2 dwarf gouramis

12. Special Considerations for Small Tanks Under 30 Gallons

It can be especially challenging to avoid overstocking smaller tanks like 10 or 20 gallon systems. Some important tips for nano tanks include:

  • Choose just one centerpiece fish like a betta or gourami rather than multiple larger species
  • Focus on nano fish species that stay under 1.5 inches size like rasboras, tetras, danios, celestial pearl danios, and CPDs
  • Avoid territorial cichlids like convicts that require more space
  • Use a sponge filter or other low-flow filtration designed for small tanks
  • Perform regular partial water changes of 30-50% to dilute nitrates
  • Add floating plants to help absorb excess nutrients and provide shelter

With careful selection of fish, tankmates, and maintenance routines, small tanks can be suitable homes for a limited number of appropriate fish species.

13. The 1 Inch Per Gallon Rule Explained

The old adage of allowing 1 inch of adult fish length per gallon of tank size is a useful starting point, but should be applied flexibly. Few modern experts recommend rigid adherence to this rule. Some important caveats include:

  • Works better for smaller fish under 3 inches than large fish
  • Doesn’t account for fish height/width – a 8″ pleco needs more than 8 gallons
  • Bigger fish have a higher bioload than small fish of the same length
  • Schooling fish should be kept in proper group sizes regardless of length
  • Doesn’t consider behaviors, compatibility, tank dimensions
  • Assumes tank is extremely heavily filtered

The inch per gallon concept provides a rough estimate but should be adjusted based on all aspects of your tank and fish selections. Use it as a starting point along with other critical factors for stocking community tanks.

14. Stocking Recommendations by Tank Size

As a very general guideline, here are some tank size capacity recommendations and average stocking levels:

Tank Size Stocking Recommendation
5-10 gallons 1 betta or 1-2 small fish species like guppies
10-20 gallons 10-15 small fish 1-2 inches length
20-30 gallons 10-20 tetras/rasboras + 5-6 cory cats or similar bottom fish
40-55 gallons 20-25 schooling fish + 5-10 bottom feeders
75-100 gallons 25-35 assorted community fish

Remember these are just approximations and tank stocking depends on the species, size, behaviors, filtration, maintenance, aquascape, and other specifics of your particular setup.

15. Signs Your Tank is Overstocked

Some clues that your tank has too many fish include:

  • Rapid nitrate accumulation between water changes
  • Fish gasping near water surface
  • Aggressive behavior and fighting over territory
  • Abnormally rapid growth or stunting
  • Fish loss soon after adding new tankmates
  • Disease outbreaks
  • Lethargy, clamped fins, or loss of appetite
  • Low oxygen levels
  • Poor plant growth
  • Dirty tank water

Pay close attention for these warning signs of overcrowding and be prepared to rehome fish if necessary. Regular testing for ammonia, nitrite, and nitrate can also diagnose overstocking issues.


Stocking fish appropriately to avoid overcrowding takes research, planning, an understanding of the nitrogen cycle, and attention to tank conditions. Focus on selecting species suitable for your tank’s size and filtration capacity. Start conservatively with a moderate stocking level and build up slowly. Provide plenty of open swimming space and territories. Test water parameters routinely and perform frequent water changes in heavily stocked tanks. Follow these best practices tailored to your specific tank size and inhabitants to enjoy a healthy, thriving freshwater aquarium.

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