Question 1

How many gallons is my fish tank?

Accepted Answer

Measure the inside of the tank in inches — length, width and height — and multiply them together, then divide by 231 (cubic inches per US gallon). A 20 × 10 × 12 inch tank is 2,400 in³ ÷ 231 = 10.4 gallons. Measure inside the glass, not outside: on a larger tank the glass thickness and trim can shave off a gallon or more compared to the advertised size.

Question 2

How many fish can I keep per gallon?

Accepted Answer

The old rule of thumb is 1 inch of fish per gallon — but it only applies to small, slim-bodied fish like tetras and rasboras, counted at full adult size, in a filtered and cycled tank. Deep-bodied or messy fish (goldfish, cichlids, plecos) need far more room than the rule suggests. Treat it as a rough ceiling, subtract 10–15% of volume for substrate and decor, and research the actual space, temperament and group-size needs of each species.

Question 3

How much does a full aquarium weigh?

Accepted Answer

Water weighs 8.34 lb per US gallon, and with the glass tank, stand, substrate, rock and equipment a good planning figure is about 10 lb per gallon total. A 55-gallon setup therefore weighs roughly 550 lb — as much as a large refrigerator. Place big tanks across floor joists near a load-bearing wall, and never on a piece of furniture not built for the load.

Question 4

What are the standard US aquarium sizes?

Accepted Answer

Common standard tanks: 10 gallon (20 × 10 × 12 in), 20 gallon high (24 × 12 × 16 in), 29 gallon (30 × 12 × 18 in), 40 gallon breeder (36 × 18 × 16 in), 55 gallon (48 × 13 × 21 in) and 75 gallon (48 × 18 × 21 in). If your measurements land close to one of these, you almost certainly have that standard size — manufacturers round the nominal gallons up slightly.

Question 5

Why does my tank hold less water than its rated size?

Accepted Answer

Three reasons: ratings use outside dimensions while water sits inside the glass; you never fill to the very rim; and substrate, rock, wood and equipment displace water. Together these typically cost 10–15% of the nominal volume, so a "20 gallon" tank often holds 17–18 gallons of actual water. Use the effective volume when dosing medication or fertilizer — overdosing a smaller real volume is a common mistake.

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