How many fish in a 20 gallon tank: Max inches

Counts by species in a 20 gallon long
Species	Adult size	Count by the rule	Practical call	Notes
Neon tetra	1.5 in	10	One school of 6 to 10	Schooling fish: keep 6+. Prefers soft, stable water.
Ember tetra	0.8 in	20	One school of 6 to 10	Schooling fish: keep 6+. Tiny, so the inch rule overstates how many you need to fill a tank.
Harlequin rasbora	2 in	8	One school of 6 to 8	Schooling fish: keep 6+.
Chili rasbora	0.8 in	20	One school of 8 to 12	Schooling fish: groups of 8+ show best. Very small mouths.
Zebra danio	2 in	8	One school of 6 to 8	Fast schooling fish: keep 6+. A long tank suits them better than a tall one.
Cherry barb	2 in	8	One school of 6 to 8	Keep 6+. Peaceful, but needs more swimming room than the inch rule suggests.
Male guppy	1.5 in	10	Up to 10	Keep males only or watch breeding: females and males together multiply fast.
Endler's livebearer	1 in	16	Up to 16	Same breeding caution as guppies.
Platy	2.5 in	6	Up to 6	Hardy, but livebearers breed. Plan for fry.
Panda cory	2 in	8	One school of 6 to 8	Bottom dweller: keep 6+ and use smooth sand, not gravel.
Bronze cory	2.5 in	6	One school of 6	Keep 6+. Needs floor space, so length and width matter more than height.
Otocinclus	1.75 in	9	One school of 4 to 8	Keep 4+. Needs a mature tank with algae to graze; often starve in new tanks.
Honey gourami	2 in	8	2 (single fish or a pair)	Peaceful, often kept as a single fish or a pair.
Dwarf gourami	3.5 in	4	2 (single fish or a pair)	One male per tank. Prone to disease in many stores; buy carefully.
Pearl gourami	4.5 in	3	Too small. Usually 30+ gal	Calm but big and tall-finned. Plan on 30+ gal whatever the inch count says.
Kuhli loach	4 in	4	Rule allows 4; group of 5+ wanted	Keep 5+. Thin and shy, but needs a long, secure tank with lids and soft substrate.
Betta	2.5 in	6	1 (one per tank)	One per tank (males fight). Each betta differs on tank mates.
Bristlenose pleco	5 in	1 (3 gal/in)	Too small. Usually 29+ gal	Heavy waste producer. The inch rule overstates capacity in small tanks.
Angelfish	6 in	0 (3 gal/in)	Too small. Usually 30+ gal	Tall body: 30+ gal is common guidance, and taller tanks suit them. Will eat small tetras.
Boesemani rainbowfish	4 in	4	Too small. Usually 55+ gal	Active schooling fish, 6+. Needs length: 4 ft tanks are the usual minimum.
Cherry shrimp	1.25 in	12	One school of 6 to 10	Minimal bioload, so the inch rule understates how many a tank holds. Colonies self-limit.

20 gal long (30 × 12 × 12 in): 18.7 gal gross, 16.5 gal net at 12%. Counts use one species alone, a medium filter, 1 gal per inch under 5 in and 3 gal per inch at 5 in or more. Adult sizes vary, and the counts are a ceiling. Do not add up two rows: the rule counts every fish in the tank together.

20 gallon high vs 20 gallon long

Tank	Net gal	Max in by gallons	Surface (sq in)	Max in by surface, tropical	Max in by surface, goldfish
20 gal high (24 × 12 × 16)	17.6	17.5	288	24	9.5
20 gal long (30 × 12 × 12)	16.5	16	360	30	12

The two shapes hold nearly the same water, but the long tank has 25% more surface. For active fish and anything at the 12 sq in per inch limit, pick the long one.

How to use this calculator

Pick your shape
Choose 20 gal long (30 × 12 × 12 in) or 20 gal high (24 × 12 × 16 in). They hold about the same water but differ in surface area.
Set the adult size
Use the species table to find your fish's adult length, then pick custom and enter it.
Check the surface-area line
The result shows an oxygen check: 12 sq in of surface per inch of tropical fish. A 20 long passes with room to spare.

The rule we use

For this page, the calculator applies the rule that matches the fish, and takes 12% of the gross gallons off by default for substrate and decor. The three rules are 1 gal per inch for small slim community fish, 3 gal per inch for goldfish and fish of 5 in or more, and 5 gal per inch for saltwater. See aquarium stocking rules explained for the formula, filtration factors, and the origin of each.

These are rules of thumb used by hobbyists, not science. Both 20 gal shapes land within 1 in of each other by the gallon rule. The surface-area rule favors the 20 long. If the two rules disagree, use the smaller number.

Common mistakes

Assuming 20 high and 20 long are equivalent
The 20 long has 360 sq in of surface and a longer swimming lane. The 20 high has 288 sq in and suits fewer active fish.
Mixing a bottom-dwelling school with active danios
Corydoras need floor space, and danios use the whole length. Check the footprint, not only the gallons.
Stocking to the number on day one
Fill a 20 gal in three or four batches over a couple of months.

Frequently asked questions

How many fish can I put in a 20 gallon tank?

About 16 in of small fish in a 20 long by the one-inch rule at 12% deduction and a medium filter. That is around 10 neon tetras, or 8 harlequin rasboras.

Is a 20 gallon high or long better?

For fish, the 20 long. It has more surface area for gas exchange and more swimming length. The 20 high suits tall-bodied fish like a single angelfish only marginally; most would prefer a 29.

How many goldfish in a 20 gallon?

One fancy goldfish, by common guidance of about 20 gal for the first fish, quoted against the nominal size. That holds for both a 20 high and a 20 long. A 29 or 40 is a better start because it leaves room for a second.

How many guppies in a 20 gallon?

Males only: 8 to 10 is typical. A mixed group breeds quickly and overstocks the tank within months.

Can I keep a school plus bottom fish in a 20 long?

Yes. One school of 8 harlequin rasboras and 6 panda corydoras uses 28 in by the rule, which is above this tank's limit. Cut the schools to fit; the table lets you check each species.

How many fish in a 20 gallon tank?

20 gallon high vs 20 gallon long

How to use this calculator

Pick your shape

Set the adult size

Check the surface-area line

The rule we use

Common mistakes

Assuming 20 high and 20 long are equivalent

Mixing a bottom-dwelling school with active danios

Stocking to the number on day one

Frequently asked questions

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