Your tank's printed size is the dry box volume. After substrate, rocks, and the gap below the rim, actual water is roughly 85% of that. Dosing medication, fertilizer, and water conditioner off the printed number overdoses your tank — use the real water volume instead.
How to use the result
Use full volume to pick a tank and stand; use real water volume for dosing, water changes, and stocking. Heater wattage scales with volume (3–5 watts per gallon), and filters are rated by gallons-per-hour — aim for 4× your real volume per hour.
FAQ
How many gallons is my aquarium?
Multiply length × width × height in inches and divide by 231 for US gallons. A standard 36×18×18 inch tank is about 50 gallons full, ~43 gallons of actual water after substrate and displacement.
Why is my real water volume lower than the tank size?
Substrate, rocks, decorations, and the few inches left empty below the rim all displace water. Actual water is typically 80–90% of the rated tank size.