Use this link to estimate water volume then see article below:
Aquarium System Volume Calculator
I am not sure where this article originated (its been copy/pasted many times) but it is the best way to calculate water volume:
It all begins with knowing your current salinity in ppt (parts per thousand) or Specific Gravity. In our example we'll use 1.026, or 35 ppt salinity. Next, we will need to estimate the total volume. This is our starting point. In my case, I have the system referenced above. I have some live rock and corals, and my sump is about 75% full of water. Using 90 gallons for the display tank, and 15 gallons for the 3/4 full sump, that's 105 gallons. I estimate that I have about 1 cubic foot of live rock and sand in my display tank. Using a web reference called online conversions.com, I found out that a cubic foot of space is about 7.5 gallons of liquid. So if 105 gallons is displaced by about 7.5 gallons, that brings us to a little under 100 gallons. To simplify the math, let's call it 100 gallons.
OK! So we know our specific gravity is 1.026, or 35 ppt; and we've estimated total volume of water at 100 gallons. Let's move on.
Using 100 gallons as my assumption, then 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. I can use my refractometer to verify this. I used the chart below to extrapolate my SG values. The picture below is tiny, but click on it to reveal a larger view.
So I've added 5 gallons of RO/DI water to my system and wait a few hours to reach equillibrium. If my guess of 100 gallons was accurate, I would anticipate a new salinity of 1.024-1.025.
When I measure the salinity, my refractometer tells me it's actually 1.023, or 31 ppt. So instead of a 5% drop in salinity, we experienced an 11% drop. That's no problem. This tells me that my estimation was 6% off. Instead of the system volume being 100 gallons, it's was really more like 94 gallons. I know I'm more like 94, and not 106 gallons because a greater movement in salinity was effected by a lesser volume of water to start with. Make sense? I know my guess was too high because my salinity drop was greater than expected.
I could stop here, satisfied that my total system volume is about 94 gallons, but I could also start over--just to verify and further dial in my total system volume. If I start over I can either adjust my salinity back up to 1.026, or start with my new figure of 1.023. For me, I like 1.026, so I'm going to add some salts to the system and keep things consistent.
If I trust my math, my actual volume is 94 gallons. I can verify this the same way as above. I'll dilute my system by 5%, or 4.7 gallons. Since a gallon is 128 ounces, I determine that 4.7 gallons is about 602 ounces, or 18.8 quarts (well, close enough).
For a final review, I anticipate that after adding 4.7 gallons of water, my new salinity is going to be 1.024-1.025. If it's not, it should be very, very close. If the resulting salinity is higher than I expected, my guess was too low, and there's actually more water in the system than I thought. Conversely, a lower salinity reflects my guess was high, and there's less than 94 gallons.
Trial and error will get you there if you perservere. Math doesn't lie.
Hopefully this short article has inspired you to figure out your entire system's liquid volume. This information could be valuable if you experience an outbreak of flatworms, or if you decide that you'd like to lower your salinity by a point or two. Knowledge instills confidence. I KNOW that when I add 3.2 ounces of Solution A, my calcium level really will go from 420ppm to 440ppm. The more control we have over our captive reef system, the better the position we are in to husband our inhabitants.
