here's my 2 cents.
Next time you do a water change you could take a piece of LR
out let it drain a minute. Weight that piece on a scale. Empty about half of a 5gal bucket into the tank. Mark the water level on the bucket and then put the rock in. Mark that level on the bucket. At this point remove the LR
and let it drain a min. Place back in your tank. Then using a measuring cup, you can take water from the tank and just keep pouring until you reach the new line.
example: takes 2 cups of water for 1 lb of rock.
so 55 lbs of rock displaces 110cups of water. there are 16 cups in a gal
, so the example would displace 6.8gals of water.
Of course fiji and Marshall LR
have different densitys. So you would have to do this 2 times.
/shrug that would get you in the ballpark, or just assume what Kevin said above will probably save you time.