A 20-gallon long tank measures 30 inches wide, 12 inches tall, and 12 inches deep
A 20-gallon long aquarium is 30 inches from left to right, 12 inches from front to back, and 12 inches from bottom to top. The "long" designation distinguishes it from a 20-gallon tall (which is narrower and taller) and a 20-gallon breeder (which is wider and shorter). If you are shopping for one, those three measurements are what you need to check against your furniture or wall space.
The footprint — the space it takes up on your stand — is what matters most when you are deciding where to put it. At 30 by 12 inches, a 20-gallon long needs a surface about 2.5 feet wide and 1 foot deep. Height-wise, 12 inches of water plus the hood and light fixture usually means the top of the tank sits around 18 to 20 inches above your stand, depending on what equipment you add.
Key Takeaways
- A 20-gallon long is 30 inches wide, 12 inches tall, and 12 inches deep — the width is what sets it apart from other 20-gallon sizes.
- The footprint of 30 by 12 inches means you need a stand or surface roughly 2.5 feet long and 1 foot deep.
- The total height from floor to the top of the hood is usually 18 to 20 inches, so measure your wall or shelf space vertically as well.
- Glass thickness and trim add roughly 1 inch to the outer dimensions, so the actual footprint is slightly larger than the water dimensions.
How the 20-gallon long compares to other 20-gallon sizes
Three different tank shapes hold 20 gallons, and they look and perform very differently. The 20-gallon long is the widest and shallowest. The 20-gallon tall is narrower (24 inches wide) but much taller (24 inches), giving it a vertical profile. The 20-gallon breeder is the widest (36 inches) but shortest (10 inches tall), designed for fish that need horizontal space and shallow water.
For most fish, the 20-gallon long is the best choice because the extra width gives fish more room to swim side to side, and the moderate height makes maintenance easier. The tall version works for plants and vertical swimmers like bettas, but it concentrates waste in a smaller footprint. The breeder is ideal for cichlids and other fish that spawn on flat surfaces but is harder to light and heat evenly.
Measuring your space before you buy
Before ordering or picking up a 20-gallon long, measure the stand or furniture where you plan to put it. Measure the width, depth, and the distance from the surface to the ceiling or shelf above. Write down all three numbers so you can compare them to the tank dimensions.
Remember that the tank itself is 30 by 12 inches, but the frame and trim add about half an inch on each side. The actual footprint is closer to 31 by 13 inches. If your stand is exactly 30 inches wide, the tank will overhang slightly and may look unstable. Aim for at least 32 inches of width and 14 inches of depth to give yourself a small margin.
For height, measure from your stand surface to any shelf, light fixture, or wall above. A 20-gallon long with a standard hood and light sits about 18 to 20 inches tall. If you are using a canopy or tall light fixture, add another 4 to 6 inches. Leave at least 2 to 3 inches of clearance above the tank so you can lift the hood and reach inside without hitting your head or ceiling.
Weight and stand requirements
A 20-gallon long filled with water, gravel, and decorations weighs between 200 and 250 pounds. The water alone is about 167 pounds (20 gallons × 8.3 pounds per gallon), and gravel, rock, and the glass tank add the rest. Your stand must support at least 300 pounds to be safe — most aquarium stands are rated for this, but furniture stands are often not.
Check the weight rating on any stand you use. If you are unsure, a dedicated aquarium stand is the safest choice. Placing a 20-gallon long on a bookshelf, dresser, or table not designed for aquariums can lead to collapse and serious damage to your floor and home.
Choosing between a 20-gallon long and other sizes
If 30 inches of width fits your space but you want more water volume, a 40-gallon breeder (36 by 18 by 18 inches) holds twice as much and is only 6 inches wider. If you need something smaller, a 10-gallon tank (20 by 10 by 12 inches) is half the footprint. If you want the long profile but more height for plants, a 29-gallon (30 by 12 by 18 inches) uses the same width and depth but adds 6 inches of height.
The 20-gallon long is popular because it balances width, height, and volume in a way that works for most fish and most homes. It is large enough to keep a small community of fish or a single larger fish comfortably, but small enough to fit in a bedroom or living room without dominating the space.
Standard dimensions across brands
The 20-gallon long dimensions are standardized across manufacturers. Whether you buy from Aqueon, Marineland, Tetra, or a local aquarium shop, a 20-gallon long is always 30 by 12 by 12 inches inside. The glass thickness, frame material, and trim style vary slightly, but the water volume and footprint are the same.
This standardization means you can shop around for the best price or features without worrying that one brand's "20-gallon long" is a different size than another's. It also means stands, hoods, and filters designed for a 20-gallon long will fit any 20-gallon long tank you buy.
Frequently Asked Questions
Is a 20-gallon long the same as a 20-gallon tall?
No. A 20-gallon long is 30 inches wide and 12 inches tall. A 20-gallon tall is 24 inches wide and 24 inches tall. Both hold 20 gallons, but the long is wider and better for fish that swim horizontally, while the tall is better for plants and vertical swimmers.
Can I put a 20-gallon long on a regular desk or table?
Only if the desk or table is rated to hold at least 300 pounds. A filled 20-gallon long weighs 200 to 250 pounds, and you need safety margin. Most office desks and furniture are not strong enough. A dedicated aquarium stand is the safest choice.
How much space do I need above a 20-gallon long for the hood and light?
A standard hood and light add about 4 to 6 inches of height. The tank itself is 12 inches tall, so the total is usually 16 to 18 inches. Leave at least 2 to 3 inches of clearance above that so you can lift the hood and reach inside.
Will a 20-gallon long fit in a corner?
Yes, if the corner is at least 30 inches wide and 12 inches deep. Measure carefully, because corners are often narrower than they look. Also make sure you can reach the back of the tank for cleaning and maintenance — if the tank is pushed all the way into the corner, you may not be able to access it.
What is the difference between a 20-gallon long and a 20-gallon breeder?
A 20-gallon long is 30 by 12 by 12 inches. A 20-gallon breeder is 36 by 18 by 10 inches. The breeder is wider and shallower, designed for fish that spawn on flat surfaces. The long is narrower and taller, better for general community tanks.