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20 Gallon Long Tank Dimensions
A standard 20 gallon long aquarium measures about 30 × 12 × 12 in (76 × 30 × 30 cm) on the outside. Exact sizes vary by brand: Aqueon lists 30¼ × 12½ × 12¾ in and Marineland 309⁄32 × 129⁄16 × 12¾ in. Once it's set up with 2 inches of sand, it holds roughly 15 gallons of water and weighs about 190 lb in total.
20 gallon long tank calculator
Drawn to scale from your numbers: sand at the bottom, water up to your fill line.
Setting up a 20 long?
- Plan your fish: the stocking calculator checks numbers and tank mates
- Size your heater for your room temperature
- Check another tank's volume, any shape
Not every 20 gallon long is the same size
The “20 gallon long” label is a class of tank, not an exact measurement. Here is what each source actually lists:
| Source | L × W × H (in) | Empty | Full |
|---|---|---|---|
| Aqueon size chart | 30¼ × 12½ × 12¾ | 25 lb | 225 lb |
| Marineland ML10202 (dealer listing) | 309⁄32 × 129⁄16 × 12¾ | 25 lb | 192 lb |
| SaltwaterAquarium.com size table | 30 × 12 × 12 | 25 lb | 225 lb |
Checked 24 Sep 2026. Marineland's full weight is simply the label gallons × 8.34 lb plus the empty tank; Aqueon's is closer to the full outside volume filled with water. Neither includes sand, rock or the stand. That's why the two brands disagree.
Same stand, two tanks
The 20 long and the 29 share one 30¼ × 12½ footprint. Only the height changes, so the same stand, glass lid and 30 in light fit both.
| Tank | L × W × H (in) | Outside volume |
|---|---|---|
| 20 gallon long | 30¼ × 12½ × 12¾ | 20.9 gal |
| 29 gallon | 30¼ × 12½ × 18¾ | 30.7 gal |
Aqueon size chart.
That makes upgrading from a 20 long to a 29 cheap: keep the stand and lid, change only the tank.
20 long vs 20 high
They hold about the same water but are built for different things:
| Aqueon | L × W × H (in) | Footprint | Outside volume |
|---|---|---|---|
| 20 long | 30¼ × 12½ × 12¾ | 2.6 sq ft | 20.9 gal |
| 20 high | 24¼ × 12½ × 16¾ | 2.1 sq ft | 22.0 gal |
- 20 long: 6 in more length and 25% more floor space. Better for bottom dwellers and small schooling fish, and easier to reach the bottom when planting or cleaning.
- 20 high: 4 in taller and a smaller footprint. It fits narrower furniture and gives tall plants more room, but less swimming length.
How much water a 20 gallon long really holds
Multiply the outside dimensions of an Aqueon 20 long and you get 20.9 gallons. You never get that much water, because of three things:
- Glass. With about 3/16 in glass (an estimate; check your tank), the inside is roughly 29⅞ × 12⅛ in.
- The gap at the top. Most people keep the water about an inch below the rim.
- Substrate. Two inches of sand takes up about 12 litres of space.
That leaves about 18.1 gallons with a bare bottom and 15.0 gallons with 2 in of sand. Use the real figure, not the label, when you dose medication or water conditioner.
Weight and stand
A set-up Aqueon 20 long comes to about 192 lb: 25 lb of glass, about 125 lb of water and about 42 lb of sand, before rock and the stand itself. That sits on 2.6 square feet.
- A set-up 20 long weighs about 192 lb. Use a stand or furniture rated above that.
- A 30 × 12 stand also fits a 29 gallon later, so buy one rated for the 29 (about 330 lb per Aqueon) if you might upgrade.
General guidance, not structural advice.
Equipment for a 20 gallon long
| Item | Starting point | How we got it |
|---|---|---|
| Heater | 45–75 W | 3–5 W per gallon of real water; use the higher end in a cold room. |
| Filter | 60–90 gph | Turning over the water 4–6 times an hour |
| Sand, 2 in deep | ≈ 42 lb | Inside floor area × depth at about 1.6 kg per litre (estimate) |
| Light | 30 in | Match the tank length |
| Lid | 30 × 12 in | Check the maker's fit list |
Rules of thumb, not guarantees. Fish choice, room temperature and filter type all change the right answer.
What a 20 gallon long is good for
The long, shallow shape suits small schooling fish, shrimp and small bottom dwellers that use floor space more than height. Check any mix in the stocking calculator.
How the numbers are worked out
Worked examples
Aqueon 20 long, 3/16 in glass, 1 in gap, 2 in sand: inside 29.88 × 12.12 in, water height 12.75 − 0.1875 − 1 − 2 = 9.562 in. 29.88 × 12.12 × 9.562 = 3,464 in³ ÷ 231 = 15.0 gal (56.8 L).
Marineland 20 long, 3/16 in glass, 1 in gap, 2 in sand: inside 29.91 × 12.19 in, water height 12.75 − 0.1875 − 1 − 2 = 9.562 in. 29.91 × 12.19 × 9.562 = 3,485 in³ ÷ 231 = 15.1 gal (57.1 L).
Bare-bottom Aqueon 20 long, 3/16 in glass, 1 in gap, 0 in sand: inside 29.88 × 12.12 in, water height 12.75 − 0.1875 − 1 − 0 = 11.56 in. 29.88 × 12.12 × 11.56 = 4,188 in³ ÷ 231 = 18.1 gal (68.6 L).
FAQ
What are the dimensions of a 20 gallon long tank?
About 30 × 12 × 12 in. Aqueon lists 30¼ × 12½ × 12¾ in and Marineland 30 9/32 × 12 9/16 × 12¾ in.
Which is longer, a 20 gallon long or a 29 gallon tank?
Neither: both are 30¼ in long and 12½ in deep (Aqueon). The 29 is 6 in taller.
What size lid fits a 20 gallon long?
A 30 × 12 in lid. The same lid fits a 29 gallon, because both tanks share the same footprint.
How much does a 20 gallon long weigh full?
Aqueon lists 225 lb and Marineland 192 lb. Our estimate with 2 in of sand is about 192 lb.
Why does a 20 long hold less than 20 gallons?
Outside dimensions include the glass, and you lose more to substrate and the gap at the top. Set up, expect about 15.0 gallons with 2 in of sand.
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Sources
- Aqueon aquarium size and weight chart, aqueon.com (PDF): www.aqueon.com
- Marineland standard aquariums, Bluefish Aquarium (dealer): bluefishaquarium.com
- Marineland 20 Gallon Long ML10202, Bluefish Aquarium: bluefishaquarium.com
- Aqueon Standard Glass Aquarium Tank 20 Gallon Long, Petco: www.petco.com
- Aquarium Weights & Sizes, SaltwaterAquarium.com: www.saltwateraquarium.com
- All accessed 24 Sep 2026. Calculations by TankRatio; “estimate” and “rule of thumb” figures are ours.