Free tool
How many gallons is your tank really
The number on the box is the empty box. This gives you the water that is actually in there, what a change is in gallons and liters, and how much salt to weigh out.
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Shape
Inside measurements in inches
What is actually in the tank
Water change
Your tank
67 gallons of water
254 liters. Empty box volume 79 gallons before fill level and rock.
20% water change
13.4 gal
51 liters to mix or condition
Salt mix for that change
4.6 lb
about 8.1 cups to reach 1.026. The half-cup-per-gallon label rule only gets to about 1.022, so mix, wait, and trim with a refractometer.
Bigger tanks are more stable and cost more per change. Both matter when you decide how often to service it.
Volume questions
The math behind the number
Multiply the inside length, width and height in inches, then divide by 231. That is the empty volume. Real water volume is lower once you subtract the space above the waterline and the rock and sand, which is why the calculator asks for both.
Ten to twenty five percent is typical. Reef tanks usually get a smaller change more often; freshwater tanks a larger change less often. What matters most is doing it on a schedule with matched water.
To reach reef salinity of 1.026, plan on roughly 0.34 pounds of dry mix per US gallon, about 40 grams per liter or a generous half cup. The half-cup-per-gallon rule printed on most labels only mixes to about 1.022. Mix, heat and circulate before it goes in, and confirm with a refractometer.
Yes. Larger tanks use more water per change and take longer, and reef tanks more than freshwater. Text Jason your gallons and a photo for an exact number.
Tell Jason about your tank
Size, saltwater or freshwater, what is bugging you. That is all he needs to give you a straight answer, usually the same day.