How many gallons of salt water will be displaced when 100 pounds of rock is submerged into a 72 gallon tank of salt water?

Answer 1

Measuring it gallon by gallon adds room for error and would take all of the fun out of my question.

Porosity would allow water in so It would not affect displacement. A simplistic example would be if you had a 1 lb gold bar and a 1 lb gold bar with a hole in it, they would displace the same amount. Wouldn't live rock be the same? Porosity would allow water inside it right?

My reason for wanting to know is for example if I have a known volume of say 100 gallons exact I know if I add 5 gallons of plain RO/DI water, (5% of the system's estimated volume) it should lower my salinity by exactly 5%. Math tells us 35 ppt lowered by 5% would yield 33.25 ppt. To review; I plan on adding 5 gallons of RO/DI water to my system, to dilute my salinity by 5%, yielding a new tank salinty of 33.25 ppt, or a SG of 1.024-1.025.

So far I have come up with 1 cubic foot of water is 7.5 gallons

Argonite is 27.216 lbs per cubic foot if my metric conversion is correct.

So 10 lbs liverock would displace .4512 cubic foot or 3.384 gallons.

http://www.nano-reef.com/forums/lofiversion/index.php/t189655.html

Answer 2

The weight of the rock (100 pounds) divided by the density difference between the rock and salt water will give you the volume of water displaced. Since salt water's density is around 64 pounds per cubic foot and rock is around 170 pounds per cubic foot, you can calculate how many cubic feet the rock displaces, then convert to gallons.