20 Gallon Tank Dimensions: 20 Long vs 20 High, Size and Weight
Published: 2026-11-14 · FishyKart Blog
A standard 20 gallon tank comes in two shapes. The 20 gallon long is 30 1/4 x 12 1/2 x 12 3/4 inches (76.8 x 31.8 x 32.4 cm) and the 20 gallon high is 24 1/4 x 12 1/2 x 16 3/4 inches (61.6 x 31.8 x 42.5 cm). Both weigh about 25 lb empty and 225 lb full according to Aqueon's aquarium dimension chart. The long version gives fish about 25% more floor area, which is why most keepers choose it.
Photo: úlfhams_víkingur - Wikimedia Commons, CC BY 2.0
20 Long vs 20 High at a Glance
| 20 gallon long | 20 gallon high | |
| Inches (L x W x H) | 30 1/4 x 12 1/2 x 12 3/4 | 24 1/4 x 12 1/2 x 16 3/4 |
| Centimetres | 76.8 x 31.8 x 32.4 | 61.6 x 31.8 x 42.5 |
| Footprint | about 378 sq in (2.6 sq ft) | about 303 sq in (2.1 sq ft) |
| Weight empty / full | 25 lb / 225 lb | 25 lb / 225 lb |
| Same footprint as | 29 gallon (18 3/4 in tall) | 15 gallon (12 3/4 in tall) |
| Best for | bottom dwellers, shrimp, active schools | tall plants, small footprint |
Dimensions and weights from the Aqueon critical dimensions chart; footprint and cm are our conversions. Other brands differ by a fraction of an inch, so measure your stand against the actual tank.
How Much Water Is Really in a 20 Gallon?
The outside box of a 20 long is about 4,821 cubic inches, or 20.9 gallons; a 20 high is about 5,077 cubic inches, or 22 gallons. Glass thickness, the plastic rim and filling about an inch below the top take that down, and gravel and decor remove more, so a set-up 20 gallon usually holds around 16-18 gallons of water. At about 8.34 lb per US gallon of water (USGS), that is roughly 135-150 lb of water; gravel, decor, glass and stand-top equipment bring Aqueon's full weight to 225 lb. Measure your own tank with our aquarium volume calculator.
Why the 20 Long Wins for Most Fish
Fish use length and floor area, not height. The 20 long is 6 inches longer than the 20 high and has about 25% more bottom area, which matters for:
- Bottom dwellers such as corydoras and kuhli loaches.
- Shrimp colonies, which spread over the bottom.
- Active schools of small tetras or rasboras that need swimming length.
- One adult axolotl: a 20 long is the usual minimum per adult in our axolotl tank setup guide.
The 20 high is a good choice where space is tight, for a single betta with tall plants, or as a display for a few slow fish. It is also easier to reach the bottom of a 20 long: 12 3/4 inches deep versus 16 3/4.
What Fits in a 20 Gallon
A 20 long can hold a small community: a school of 8-10 small tetras or rasboras, a group of 5-6 corydoras, and a centrepiece such as a dwarf gourami or a betta, with snails or shrimp. Stocking depends on bioload, not a fixed number of inches, and on how often you clean; our lists are in best fish for a 20 gallon tank. Big fish such as oscars, goldfish groups or plecos that grow over 6 inches need much larger tanks.
Stand and Placement
At 225 lb, a full 20 gallon is too heavy for most bookshelves or dressers. Use a stand made for the footprint: 30 x 12 inches for a 20 long, 24 x 12 inches for a 20 high. The tank must sit flat with no overhang; a bowed shelf or uneven floor puts stress on the seams. Keep it away from windows and heaters, and leave space behind for the filter and cables. Moving up later? Compare with the 29 gallon, which has the same footprint as a 20 long, or the 40 gallon breeder.