A gallon label is a size name, not a floor size. A taller tank holds more on the same floor, so a bigger label can sit on less floor.
Floor size
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20 High20 Long24¼ inlong30¼ inlong
Aqueon’s two 20 gallon tanks, the 20 High and the 20 Long, are both 12½ in wide. The 20 Long is 6 in longer, so it stands on more floor.
Same floor, different labels
Aqueon’s 20 Long and 29 are both 30¼ x 12½ in from above. The 29 is 6 in taller, at 18¾ in against 12¾. Its 60 Breeder, 75 and 90 share a 48½ x 18½ in floor, and its 30, 38 and 45 share 36¼ x 12⅝ in.
Zoo Med’s 28 gallon Low Boy and 40 gallon terrariums are both 36 x 18 in, one 10 in tall and the other 18. Aqueon’s 40 Breeder is within ¼ in of that floor, at 36³⁄₁₆ x 18¼ in.
A bigger label on less floor
Aqueon’s 40 Breeder has about 660 square inches of floor, and its 55 about 615. The 55 is about a foot longer but 5½ in narrower.
REPTI ZOO calls its 18 x 18 x 24 in terrarium a 30 gallon and lists its volume as 33.6 gallons. That’s close to the whole outside box: 18 x 18 x 24 in is 7,776 cubic inches, or 33.7 US gallons at 231 cubic inches a gallon. Zoo Med sells the same size as its NT-4 with no gallon label, and Aqueon’s 20 Long, a smaller label, has more floor.
Why the floor matters
For fish, the water’s surface is where oxygen gets in. Wide tanks are better than tall ones because they have more surface for the volume of water, and bigger tanks keep their temperature and water conditions steadier.
To compare two tanks, multiply each one’s length by its width, and measure the space it will stand in the same way. Every label’s size is in the fish tank sizes chart.
Common questions
Is a 20 long or a 20 high bigger?
They carry the same label, but the 20 Long has more floor. Aqueon's 20 Long is 30 1/4 x 12 1/2 in, about 378 square inches, and its 20 High is 24 1/4 x 12 1/2 in, about 303, and 4 in taller.
How do I work out gallons from a tank's size?
Multiply the length, width and height in inches and divide by 231 for US gallons. With outside sizes that's the whole box, which is more than the water the tank holds, because the glass and frame take up room.
Sizes come from Aqueon’s dimensions sheet and Marineland, Zoo Med, Exo Terra and REPTI ZOO’s product pages, and why the floor matters from the RSPCA. Checked against published data from Zoo Med, Exo Terra, Aqueon, Marineland and REPTI ZOO (plus 1 more).
See all 10 sources
Aqueon, “Critical dimensions for all aquariums” (product catalog page 66, PDF, 2023)
Zoo Med, Low Boy Breeder Flat Terrarium, 28 and 50 gallon (BFT-28T, BFT-50, product page)
REPTI ZOO (official store), 30 Gallon Front Opening Terrarium 18 x 18 x 24 in (RK0110, product page)
RSPCA, Ornamental fish: Environment (“Choosing an aquarium”)
General care guidance from animal welfare charities, vets and makers' published sizes. It isn't veterinary advice: if your pet seems unwell or has eaten something it shouldn't, call your vet.
