How to calculate Water volume with Rock

[Viking Dr]

When I filled my tank, I didn't pay attention to the amount of water I put in after I had my Rock where I wanted it. Ya ya I know..stupid!!!

How do I calculate the volume in gallons, when I have a 90 gall tank and 92 LBS of rock! Does the volume of the rock matter or is it just 8.35lbs/1 gal water so 92 lbs of rock = 11.02 Gallons of water displaced by rock. 92/8.35=11.02 gal. The reason I am wondering is for dosing and medication purposes.

Is this correct on my part?

[Pete H.]

No, 11 gallons is not quite right. In order to get that you are assuming the live rock has the same density as water. We know this is not true because the rock all sinks to the bottom meaning it is more dense then water. I would say you are probably looking at closer to 6-7 gallons of water for you live rock and not 11.

(Don't forget to add in your water volume for your sump and also subtract your volume for the sand bed - if you have these.)

Consider using this website, it might help you get a basic idea of your water volume, at least in a ballpark scale:

http://reef.diesyst.com/volcalc/volcalc.html

[seapug]

The only way to know for absolute sure is to drain the tank and count the gallons, but using an alkalinity test kit and the Reef Chemistry Calculator you could determine the volume by seeing how much a set amount of baking soda raises your alkalinity, but there's still a lot of ways to get inaccuracies with that.

http://reef.diesyst.com/chemcalc/chem_calc3.html

Personally, I visually guesstimate the volume of rock and sand compared to the capacity of the tank and add an estimate of the volume of water in the sump, plus a few gallons more for "pore water" in the interior of rocks and sand. I then shave off a few gallons to err on the safe side in dosing. After you get into a routine and determine calcium and alkalinity consumption rates vs. supplement rates, you can get a pretty good estimate of your actual water volume.

[Dustin1300]

Two ways:

1) Take each rock out one by one and fill bucket full and measure amount of displacement using the displacement theory.

2) Weigh the tank and subtract weight of stand, tank, rock and then divide that number by 8.35 lbs.

Really no easy way but would not be too concerned with knowing the EXACT amount of water volume. You'll find out with dosing how much the water volume is and adjust accordingly.

[codyreef29]

take all the rock outta the tank and smash it all to dust then put it in buckets and see how much water is displaced. the buy gallons on silicone or quickcrete get the volume of that remake all your rock then you will know the exact water displacement of your tank.

[Darth Vedder]

I knew I would want an accurate volume so after my dry rock and sand was in the tank I filled my system by making a 4 gallon mark on a bucket and so I knew exactly how much is in the system

But as mentioned above, ou could determine the volume of rock by measuring water displacement

[radicaltimes]

I have a 120 gallon with a 30 gallon Sump. My LFS figured with my 250 pounds of live rock I have 100 gallons of water. Even if the rock is heavierthan water it is porous

[Sk8r]

Scratch the medication purposes. Never ever ever ever ever medicate a reef! you'll kill all your bacteria. For dosing purposes, you're usually ok: they'll tell you try a couple of tsp per 50 gallons, and you just figure your volume plus sump as accurate enough for most calcium/magnesium purposes. Anything more exotic than that, you handle by doing 10% water changes weekly or 20% a month.
You need a separate completely bare qt tank for meds, adequate for the largest fish you'll ever buy.