For freshwater aquarium salt, enter the dose from your box, commonly one tablespoon per five gallons, and get the amount for the water you are treating. Many plants and some fish, such as scaleless catfish, do not tolerate salt well, so check first.
How tank volume is worked out
Every shape comes down to base area × height. For a rectangular tank the base is length × width. A cylinder uses π × radius², a regular hexagon uses 2.598 × side², and a quarter-circle corner tank uses a quarter of π × radius². A bow front is treated as a rectangle plus a curved segment whose area is two thirds of the length times the extra depth at the centre of the bow, which matches most commercial bow fronts closely. In inches the result is divided by 231 to give US gallons; in centimetres it is divided by 1,000 to give litres.
The nominal size of a tank is rarely the water it holds. TankMath subtracts the substrate depth and the gap you leave below the rim, then a percentage for rock and decor, to estimate the real water volume. That figure drives the water change, dosing and schedule, because medicine, conditioner and salt should be measured against the water actually in the tank. The filled weight adds water at about one kilogram per litre and substrate at about one and a half.
Water changes, conditioner and salt
Regular partial changes keep nitrate low and water chemistry steady. With a 25% weekly change, only about a third of today’s water is still in the tank four weeks later. Water conditioner doses scale directly with volume, so the calculator multiplies the label rate by the gallons you are treating. For marine mixes, specific gravity is converted to salinity in parts per thousand and multiplied by the litres you are making; because brands differ, always confirm with a refractometer or hydrometer before the new water goes in.
A schedule that fits your tank
The maintenance plan scales with stocking: heavier stocking means larger, more frequent water changes and more frequent filter rinses. Planted tanks get trimming and fertiliser reminders and gentler substrate cleaning, and saltwater tanks get daily top-offs with fresh RO water and weekly tests. Everything is calculated in your browser and saved only on this device.
Frequently asked questions
What specific gravity should a reef tank be?
Most reef keepers aim for 1.025–1.026. Fish-only tanks are often kept a little lower, around 1.020–1.024.
How much salt per gallon for saltwater?
Roughly half a cup, or about 130–135 g, per US gallon for 1.025, but mixes vary, so measure with a refractometer.
Should I add salt when topping off?
No. Evaporation removes only water, so top off with fresh RO water.
Is TankMath free?
Yes. Every calculator is free with no account.
Is my tank information saved?
Only in this browser, so the calculator remembers your tank next time. Nothing is uploaded.