125 gallon

Key stats

How many gallons is a 72 × 18 × 21 in tank?

A 72 × 18 × 21 in aquarium contains 118 US gallons by outside dimensions. The math:

72 × 18 × 21 = 27216 in³
27216 ÷ 231 in³/gal = 117.82 US gal
117.82 × 3.785 = 446.0 L

In practice, actual water capacity is 108 gallons after subtracting 0.375" of glass thickness on each side. Manufacturers typically market this as a 125-gallon tank for round-number simplicity.

For a different shape (cylinder, hex, bow-front) or to enter custom dimensions, use the full aquarium volume calculator.

How much does a filled 72 × 18 × 21 in tank weigh?

• Saltwater reef (with ~108 lb live rock + 1" sand bed): 1293 lb (586 kg)
• Freshwater (with 20 lb gravel, no rock): 1107 lb (502 kg)
• Empty glass only: approximately 171 lb
• Floor load: 144 lb/ft² (saltwater filled)

Floor-load warning: at 144 lb/ft², this tank exceeds the 40 lb/ft² typical residential live-load rating. Place over a load-bearing wall or get a structural assessment before installing upstairs.

Full breakdown including equipment, sump, and custom rock weight: aquarium weight calculator.

Glass thickness for a 72 × 18 × 21 in tank

Using the Wisner hobby formula with a 3.8 safety factor, this tank requires:

• Annealed (regular float) glass: 15 mm minimum (0.591")
• Tempered glass: 8 mm minimum (tempered is ~4× stronger than annealed)

Most commercial tanks at this size ship one thickness step up for additional safety. See the glass thickness calculator for the full formula and Starphire (low-iron) options.

Heater wattage for a 72 × 18 × 21 in tank

In a warm room (~70 °F ambient) targeting 78 °F, this size tank needs about 324 W of heating. The conservative reefkeeping practice is to run two heaters at 175 W each: if one fails stuck-on, the other isn't powerful enough to cook the tank alone; if one fails stuck-off, the remaining one slows the temperature drop while you replace it.

For cold-room or cool-basement adjustments: heater wattage calculator.

How much sand or gravel for a 72 × 18 × 21 in tank?

• 2" sand bed: ~128 lb of CaribSea Special Grade aragonite (7 × 20-lb bags)
• 4" deep sand bed (DSB): ~255 lb
• 1" shallow / aesthetic: ~64 lb

Other substrate brands (Nature's Ocean, Tahitian Moon, Eco-Complete): substrate calculator.

FAQ

How many gallons is a 72 × 18 × 21 in tank?

A 72 × 18 × 21 in aquarium holds approximately 118 US gallons by outside dimensions (446.0 L). After subtracting a standard 0.375" glass thickness on each interior wall, the actual water capacity is about 108 US gallons. Manufacturers commonly market this as a "125-gallon" tank.

How much does a filled 72 × 18 × 21 in tank weigh?

A 72 × 18 × 21 in saltwater reef tank with ~108 lb of live rock and a 1" sand bed weighs about 1293 lb (586 kg) filled. Freshwater with no rock comes in lighter, around 1107 lb. Floor load is roughly 144 lb per square foot — most residential floors are rated for 40 lb/ft² live load, so confirm before placing upstairs.

What glass thickness do I need for a 72 × 18 × 21 in tank?

Using the Wisner hobby formula with a 3.8 safety factor, a 72 × 18 × 21 in tank needs at least 15 mm annealed (regular float) glass. Tempered glass can be 8 mm and still meet the same safety margin. This is the calculated minimum — most commercial tanks at this size ship with one step up for safety. See the glass thickness calculator for the full math.

What size heater for a 72 × 18 × 21 in tank?

For a tank in a warm room (~70 °F ambient), this size needs roughly 324 W of heating. For redundancy, run two heaters at 175 W each — heaters fail, and a single point of failure on tropical fish or coral is expensive.

How much sand or substrate for a 72 × 18 × 21 in tank?

A 2-inch sand bed in this footprint (72" × 18") needs approximately 128 lb of CaribSea Special Grade aragonite, or about 7 20-lb bags. For a deep sand bed at 4 inches, double that.

Is a 72 × 18 × 21 in tank good for a reef?

Yes — the 72 × 18 × 21 in footprint is well-suited to a reef tank. The width gives enough front-to-back depth for aquascaping. Height is in the SPS-friendly range for most fixtures.

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