Live Rock=Water Displacement

[rick s]

I'm just curious: I know there are many varieties of live rock, some more dense, some less. But, on average, rough estimate, how much water does live rock displace?

In my 90 gallon tank (tank only) with 120lbs vanuatu/fiji rock, how many gallons of water are in the tank?

Thanks,

[jdieck]

Live Rock (Similarly Limestone) vary a lot in density from as low as low as 110 Lbs/ft3 to higher than 160 lbs/ft3.
For aquarium calculation purpose I usually consider around 130 for the lighter one and around 140 lbs/ft3 for the heavier one and usually have come close.
So assuming the 140 lbs/ft3 ten pounds of Live Rock will displace approximately 1/2 gallon of water (0.53 to be more precise). Or 18.72 pounds of rock will displace one US gallon of water.

Similarly Sand density varies but a good range will be between 95 to 105 lbs/ft3.
Using 100 lbs/ft3 the displacement will be that for every 10 pounds of sand will displace about 3/4 US gallons (0.748 to be more precise) or 13.37 pound of sand will displace one US Gallon.

In summary:
Deduct 1/2 gallon of water for every 10 pound of rock and 3/4 gallons of water for every 10 pounds of sand.

SO if your 90 gal aquarium contains around 110 pound of rock and 100 pounds of sand your actual water volume will be approximately 77 gallons.

[Randy Holmes-Farley]

In my 90 gallon tank (tank only) with 120lbs vanuatu/fiji rock, how many gallons of water are in the tank?

Not as much as you'd expect, if you count water in the pores of the rock as being water and not rock. Whether to do so or not (or to what extent) depends a bit on what you want the numbers for.

However, the actual water volume, including pore volume, is probably something like 85 gallons. That assumes that the 120 ponds is all rock, and the rock is aragonite with a density of 2.8 kg/L. So 120 pounds weighs 54.4 kg, and occupies 19 L (5 gallons) of solid rock space.

Of course, the pore water volume is not zero, so the rock is a bit puffier than that, and also carries water in it when pulled out of the tank.