Reef Tank Actual Water Volume Calculator
Your "75 gallon" tank doesn't hold 75 gallons of water. Sand and rock displace a meaningful chunk of it, and that number is what every dosing and stocking calculator should actually use.
Your Tank
Gross volume
65.5
gallons
Displaced
8.6
gallons
Sand + rock
5.6 / 3.0
Actual water volume
56.9 gallons
Use this number, not your tank's nominal size, for dosing and stocking calculators.
Why this matters: Dosing calculators that use nominal tank size overdose by the same percentage your rock and sand displace. On a typical reef tank with 1-1.5 lbs of rock per gallon, that's commonly 10-15% too much. Rock displacement here uses roughly 0.1 gallon displaced per pound, a reasonable average for typical dry/reef rock - dense aquacultured rock or heavily packed scapes will displace somewhat more.