Aquarium size determines swimming space, equipment needs and the load on the stand and floor. Check the exact manufacturer’s dimensions before buying: a nominal gallon size does not specify a universal footprint, and a filled aquarium weighs far more than the empty glass tank.
Common sizes and approximate dimensions
The dimensions below describe familiar rectangular formats in inches, rounded for comparison. They are not cabinet-fit measurements. Rims, glass thickness and manufacturer differences change the actual exterior size.
| Nominal US gallons | Approximate length × width × height | Freshwater alone, approximately |
|---|---|---|
| 5.5 | 16 × 8 × 10 | 46 lb |
| 10 | 20 × 10 × 12 | 83 lb |
| 15 | 24 × 12 × 12 | 125 lb |
| 20 high | 24 × 12 × 16 | 167 lb |
| 20 long | 30 × 12 × 12 | 167 lb |
| 29 | 30 × 12 × 18 | 242 lb |
| 40 breeder | 36 × 18 × 16 | 334 lb |
| 55 | 48 × 13 × 21 | 459 lb |
| 75 | 48 × 18 × 21 | 626 lb |
| 90 | 48 × 18 × 24 | 751 lb |
| 125 | 72 × 18 × 23 | 1,043 lb |
The water weights use nominal capacity × 8.34 lb per US gallon. They exclude the aquarium, stand, substrate, rock, equipment and other water-filled vessels. Actual water volume is lower when substrate, decorations and an unfilled top space displace water.
Aqueon’s published dimensions and weight chart is an example of the model-specific information to consult. Confirm the current specification for the actual tank being purchased, rather than transferring one brand’s empty weight to another tank.
Calculating the complete load
Start with the empty aquarium’s specified weight. Add the expected water mass, substrate and hardscape, then equipment, stand and any sump or reservoir. Avoid adding the weight of a full nominal water volume and solid decorations without recognizing that those solids displace some water.
A liter of freshwater weighs approximately one kilogram. Saltwater is somewhat denser: at specific gravity 1.025, a rough estimate is 8.34 × 1.025, or 8.55 lb per US gallon. Temperature and salinity affect exact density, so do not present a rounded planning calculation as a structural assessment.
Use a stand designed and rated for the particular aquarium. Check the manufacturer’s support requirements, including whether the base needs full support or a specified mat. A desk looking sturdy does not establish that it can support the sustained load. Have uncertain floor support assessed before installation; there is no universal gallon threshold below which every floor is safe.
Footprint matters as much as volume
A 20-gallon long gives more horizontal swimming and bottom space than a taller 20-gallon format. Height may be useful for particular animals or layouts, but it does not replace length for active swimmers or floor space for bottom dwellers.
Choose the species and social group before the tank. A length-based category such as “all fish under three inches” cannot specify a suitable minimum. Compare the 10-gallon stocking guide and the cautions for large freshwater fish instead of using an inch-per-gallon formula.
Allow room for equipment and maintenance
Measure beyond the glass footprint: leave space for hoses, cables, lids, filter removal and reaching the bottom. Check cabinet access before choosing a large canister filter or sump. A taller aquarium can be harder to plant, clean and light effectively.
Budget for suitable filtration, temperature control and lighting as well as the tank. Plan water-change access and the route into the room. Never move a filled aquarium; see the aquarium moving guide before relocating an established setup.
