Quick answer: A standard 20 gallon fish tank comes in two main shapes: the 20 long, which measures 30 x 12 x 12 inches (76 x 30 x 30 cm), and the 20 high, which measures 24 x 12 x 16 inches (61 x 30 x 41 cm). Both hold about 20 US gallons (75.7 liters) and weigh roughly 200 to 225 pounds once filled with water, gravel and decor. For most fish, the 20 long is the better buy because its footprint is 25% bigger.
That’s the short version. The longer version matters, because “20 gallon” on a box can mean five different shapes, and the one you pick decides which stand fits, which light fits, and which fish will actually be happy in it.
20 Gallon Tank Dimensions Chart (All Shapes)
Here are the common 20 gallon shapes side by side. Dimensions are the nominal size (length x width x height), which is what stores and stands are built around.
| Tank shape | Inches (L x W x H) | Centimeters | Footprint | Filled weight |
|---|---|---|---|---|
| 20 Long | 30 x 12 x 12 | 76 x 30 x 30 | 360 sq in | About 200 to 225 lb |
| 20 High (also called 20 regular or 20 tall) | 24 x 12 x 16 | 61 x 30 x 41 | 288 sq in | About 200 to 225 lb |
| 20 Extra High | 20 x 10 x 24 | 51 x 25 x 61 | 200 sq in | About 190 to 215 lb |
| 20 Hexagon | 18.75 x 16.25 x 20.6 | 48 x 41 x 52 | Narrow, 6-sided | About 220 lb |
| 60 cm rimless (often sold as “60U”) | 23.6 x 14.2 x 14.2 | 60 x 36 x 36 | 335 sq in | About 200 to 220 lb |
A few practical notes on that chart:
- Outside dimensions run slightly bigger. The glass and trim add a little. A typical 20 long from a big brand measures about 30.25 x 12.5 x 13 inches on the outside, and a 20 high about 24.25 x 12.5 x 16.75 inches. Measure your shelf against those numbers, not the rounded ones.
- Brands vary a bit. Always check the listing before buying a stand, lid or light.
- In feet: a 20 long is 2.5 ft long and 1 ft deep. A 20 high is 2 ft long, 1 ft deep and about 1.3 ft tall.
Is There a 20 Gallon Breeder Tank?
Not as a standard glass size. This trips up a lot of people. “Breeder” tanks are the wide, shallow ones, and the smallest standard breeder is the 30 gallon breeder at 36 x 18 x 12 inches, followed by the 40 breeder at 36 x 18 x 16 inches.
If you see a “20 gallon breeder” online, it’s usually a reptile terrarium (often 24 x 18 x 12 inches) or a small breeding box that hangs inside a tank. Neither is a standard fish tank. If you want a shallow, wide 20 gallon for breeding or for bottom dwellers, the 20 long is the closest thing to a breeder in this size.
How Much Does a 20 Gallon Fish Tank Weigh?
An empty glass 20 gallon weighs about 25 pounds. The water is what makes it heavy. Freshwater weighs 8.34 pounds per gallon, so 20 gallons of water alone is about 167 pounds.
| What’s in it | Approximate weight |
|---|---|
| Empty glass tank | 25 lb (11 kg) |
| Water only (20 gallons) | 167 lb (76 kg) |
| Substrate, rock and wood | 20 to 35 lb (9 to 16 kg) |
| Total, fully set up | About 200 to 225 lb (90 to 102 kg) |
That’s the weight of an adult person sitting on a small shelf around the clock. Use a stand built for aquariums, or furniture rated well above 225 pounds, and make sure it sits level on a solid floor.
20 Long vs 20 High: Which One Should You Get?
They hold the same water, but they are not the same tank.
| 20 Long | 20 High | |
|---|---|---|
| Size | 30 x 12 x 12 in | 24 x 12 x 16 in |
| Floor and surface area | 360 sq in (25% more) | 288 sq in |
| Best for | Schooling fish, corydoras, loaches, shrimp, carpet plants | Tight spaces, taller plants, a vertical look |
| Light reaching the bottom | Easier (12 in of water) | Harder (16 in of water) |
| Maintenance | Easy to reach the bottom | Deeper reach to the back corners |
| Space needed | 30 in wide surface | 24 in wide surface |
Get the 20 long if you can fit it. Fish use horizontal space far more than vertical space. The long gives schooling fish a proper runway, gives bottom dwellers more floor, and has more water surface for oxygen exchange. It’s also easier to plant and scape, since light only has to go through 12 inches of water.
The 20 high makes sense when your spot is narrow, when you want a taller layout with stem plants or vallisneria, or when you’re keeping small fish that don’t need much swimming length.
One thing to watch: a stand made for a 20 high has a 24 x 12 inch top. A 20 long won’t sit safely on it. The tank’s whole bottom edge needs full support, so buy the stand for the exact tank shape.
What About the 20 Extra High and 20 Hex?
These are the “decor” shapes. They look striking in a corner, but both have a small surface and deep water, which makes them harder to keep well.
- 20 extra high (20 x 10 x 24 in): basically two 10 gallons stacked. It works for a few calm fish and tall plants, but light struggles to reach the bottom, and cleaning the substrate needs long tools.
- 20 hex (about 18.75 x 16.25 x 20.6 in): the six-sided shape distorts the view a bit, and the small opening limits filters and lights. Stock it lightly and add surface agitation for oxygen.
If this is your first tank, skip both and go with a 20 long.
What Equipment Fits a 20 Gallon?
- Filter: rated for 20 to 30 gallons. A hang-on-back or a large sponge filter both work.
- Heater: 75 to 100 watts for tropical fish. A 50 watt heater is too weak for a 20 gallon in a cool room.
- Light: match the tank length. A 20 long needs a 30 inch fixture, and a 20 high needs a 24 inch fixture. A 20 high needs a stronger light if you want plants to grow at the bottom.
- Lid: buy it for the exact shape. Lids for the long and high are not interchangeable.
- Substrate: about 20 to 25 pounds gives a 2 inch bed in a 20 long. Aquarium soil if you’re going planted.
- Water conditioner to remove chlorine and chloramine from tap water.
Before any fish go in, cycle the tank. It takes a few weeks, and skipping it is the most common way new 20 gallons lose their first fish. Our fish tank setup guide walks through the whole process, and our aquarium volume calculator gives you the real water volume, heater size and filter flow for your exact tank.
How Many Fish Can a 20 Gallon Hold?
Forget the “one inch of fish per gallon” rule. It treats a slim tetra and a chunky goldfish the same. Stock by adult size, waste output and how the fish swims.
A realistic, well-balanced 20 long might hold a school of 8 to 10 small fish, a group of 6 bottom dwellers, and one centerpiece fish. A 20 high can hold about the same number of small fish, but give it fewer bottom dwellers since it has less floor.
Want to check your own list? Our aquarium stocking calculator scores your 20 gallon by adult size and waste, and warns you about group size, temperature and fish that don’t mix.
Best Fish for a 20 Gallon Tank
| Role | Good choices | Best shape |
|---|---|---|
| Schooling fish | Ember tetras, neon tetras, chili rasboras, galaxy rasboras, harlequin rasboras | Long or high |
| Active schooling fish | Tiger barbs (group of 6 to 8, 20 long only, no long-finned tank mates) | Long |
| Bottom dwellers | Panda corys and other corydoras, kuhli loaches | Long |
| Small bottom dwellers | Pygmy corydoras | Long or high |
| Centerpiece | Honey gourami, a betta, a pair of shell-dwelling cichlids | Long or high (shell dwellers need the long) |
| Cleanup crew | Cherry shrimp, Amano shrimp, nerite snails, ramshorn snails | Long or high |
New to the hobby? Our best aquarium fish for beginners guide covers the hardiest picks.
Ready-made stocking plans
- Peaceful community (20 long): 8 to 10 ember tetras, 6 panda corys, 1 honey gourami, 2 nerite snails.
- Nano community (20 high): 10 to 12 chili rasboras, 6 pygmy corydoras, a colony of cherry shrimp.
- Livebearers (20 long): 4 to 5 platies (all male, or be ready for babies), 6 corydoras, 1 bristlenose pleco.
- Species tank: one betta with shrimp and snails, or a small colony of shell dwellers in a 20 long.
Add fish a few at a time over several weeks so the filter bacteria can keep up.
Plants and Decor for a 20 Gallon
Live plants make a 20 gallon more stable and give shy fish somewhere to hide. Easy starters from our low light aquarium plants guide, java moss, and dwarf sagittaria for a carpet-like foreground all do well in a basic setup. For hardscape, see our fish tank decorations guide.
Common 20 Gallon Mistakes
- Buying a 20 high when the fish you want need a 20 long.
- Putting a 20 long on a stand or shelf built for a 20 high.
- Using a heater that’s too small for the room temperature.
- Adding fish before the tank has cycled.
- Overstocking because the new tank “looks empty.”
FAQ
How big is a 20 gallon tank?
A 20 gallon long is 30 inches long, 12 inches wide and 12 inches tall (2.5 x 1 x 1 feet). A 20 gallon high is 24 x 12 x 16 inches (2 x 1 x 1.3 feet). Filled, either one weighs about 200 to 225 pounds.
How long is a 20 gallon tank?
A 20 long is 30 inches (76 cm) long, and a 20 high is 24 inches (61 cm) long. Add about a quarter inch for the trim when measuring your stand or shelf.
What are the dimensions of a 20 gallon fish tank?
A 20 gallon long is 30 x 12 x 12 inches (76 x 30 x 30 cm), and a 20 gallon high is 24 x 12 x 16 inches (61 x 30 x 41 cm). Less common shapes include the 20 extra high at 20 x 10 x 24 inches and the 20 hexagon at about 18.75 x 16.25 x 20.6 inches.
What are the dimensions of a 20 gallon long tank?
30 inches long, 12 inches wide and 12 inches tall, or 76 x 30 x 30 cm. Outside measurements with the trim are about 30.25 x 12.5 x 13 inches.
What are the dimensions of a 20 gallon high (tall) tank?
24 inches long, 12 inches wide and 16 inches tall, or 61 x 30 x 41 cm. “20 high,” “20 tall” and “20 regular” all refer to this size.
How many liters is a 20 gallon tank?
20 US gallons is about 75.7 liters. After substrate and decor, the actual water volume is usually closer to 60 to 65 liters. Our aquarium volume calculator works out the exact number for your tank.
How much does a 20 gallon fish tank weigh when full?
About 200 to 225 pounds (90 to 102 kg) fully set up. The empty glass is around 25 pounds and the water alone is about 167 pounds.
Is there a 20 gallon breeder tank?
Not as a standard size. The smallest standard breeder is the 30 gallon breeder at 36 x 18 x 12 inches. For a wide, shallow 20 gallon, the 20 long is the closest match.
Is a 20 gallon good for beginners?
Yes. It’s one of the best starter sizes. The extra water makes it far more forgiving than a 5 or 10 gallon, and it still fits on most furniture that’s rated for the weight.
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