What size aquarium is 30x12x12?

At a glance

Outside dimensions

30 × 12 × 12 in · 76 × 30 × 30 cm

US gallons

18.7

UK gallons

15.6

Liters

70.8

Water weight (full)

71 kg / 156 lb

Gross volume of the glass box, calculated from the outside dimensions.

The calculation

Multiply length × width × height: 30 × 12 × 12 = 4,320 cubic inches. Divide by 231 cubic inches per US gallon and you get 18.7 gallons. In metric, the tank is about 76 × 30 × 30 cm, which is 70.8 liters. It's sold as a "20 long" because tank names are rounded up from the outside dimensions; the real capacity of the glass box is just under 19 gallons.

Inside the glass, with a 1-inch gap below the rim and a 2-inch sand bed, the water volume comes down to around 59 liters (15.6 US gallons). That's the figure to use for dosing medication and sizing equipment. You can reproduce it in the volume calculator by loading the 20 long preset and entering your substrate depth.

20 long vs 20 high: which is better?

Both tanks hold about 20 gallons, but they're very different shapes. A 20 high measures 24 × 12 × 16 inches, giving 19.9 gallons gross. The 20 long gives up a gallon of volume, but its 30 × 12 inch footprint is 360 square inches, 25% more floor space than the 20 high's 288 square inches. That extra length and surface area matters more than the extra gallon.

Fish swim horizontally, and oxygen enters the water through the surface, so a long, shallow tank supports more fish than a tall one of the same volume. The 20 long gives schooling fish six more inches to swim, gives bottom-dwellers like corydoras more sand, and its 12-inch height makes planting and maintenance easy without long tools. The 20 high suits tall plants, angelfish-shaped fish in very small numbers, or a display where height matters more than swimming room.

Weight, heater and filter

A running 20 long with glass, a 2-inch sand bed (about 19 kg or 42 lb of sand), décor and water weighs in the region of 100 kg (220 lb). Any purpose-built 20-gallon stand handles that easily, but it's too much for many shelves, bookcases and small side tables. Always place it on something rated for the load and check it's level.

For a net volume of about 59 liters held 5 °C (9 °F) above room temperature, a heater needs about 59 W, so a 75 W heater is the right choice. For filtration, a target of six turnovers an hour is about 350 liters per hour (roughly 95 GPH) of real flow. A filter rated for 30–40 gallons, or a hang-on-back filter plus a sponge filter, gives that comfortably.

What fish can live in a 20-gallon long?

The 20 long is ideal for a peaceful community of small fish. A good example: a school of 10–12 small tetras or rasboras (such as ember tetras, neon tetras or harlequin rasboras) in the middle of the water, a group of six pygmy or panda corydoras on the sand, and a few nerite snails to graze algae. Alternatively, keep a single betta with a small school of peaceful fish that won't nip its fins, or a colony of shrimp with a few nano fish.

Avoid fish that grow too big for 30 inches, including common plecos, most cichlids except dwarf species, goldfish and larger barbs. Plan around adult size, not the size in the store, and stock gradually after the tank has cycled. The stocking calculator checks your exact list for bioload, schooling numbers and compatibility.

Glass and build quality

Factory 20 longs are framed tanks, typically made with 3/16-inch (about 5 mm) glass, supported by the plastic rim. If you're building or buying a rimless 76 × 30 × 30 cm tank, the glass thickness calculator recommends 5 mm glass for the front and back and 4 mm for the ends at the standard safety factor, so the two approaches line up closely.

Related guides and tools

• Stocking Calculator
• Glass Thickness
• Fish for a 10-gallon