EDIT: This thread has been improved. The article below works--but there's now a better version. To skip to that now, CLICK HERE. Have you ever struggled with label instructions directing you to add a certain number of drops of a product, per gallon of water? It might be a medication or a supplement to add calcium. It sure would be nice to know how many drops, or how much product to add. How do we know exactly, the water volume of our reef systems? Say I have a 90 gallon aquarium and a sump constructed from a 20 gallon tank. The sum of those is 110 gallons; but, the aquarium contains sand and live rock, and the sump is not filled to the top. The protein skimmer and reactor chamber each contain some liquid. Most of us just estimate, and that gets us by--but it would be nice to determine with some precision, how much water comprises our entire system. Here's a method that you can use to determine your entire system's volume--tank, sump, skimmer, reactor, and anything else that might hold water--with a pretty good degree of accuracy. You will need an accurate salinity measuring device such as a refractometer, and a vessel such as a bucket; to accurately measure liquid volume. First, use your refractometer to find the current salinity in ppt (parts per thousand) or Specific Gravity. Mine is 1.026, or 35 ppt salinity. Next, we establish a starting point for our process. This begins with an educated guess of our total volume. I have the system referenced above; a 90 gallon display tank with a 20 gallon sump. I have some live rock and corals in the display, and it looks like my sump is about 75%full of water. A 90 gallon display and a 20 gallon sump totals 105 gallons. I estimate that I have 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, we can assume that our system contains a little under 100 gallons. To simplify the math, let's call it 100 gallons exactly. 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. If I dilute 100 gallons of saltwater with exactly 5 gallons of freshwater, my salinity will change by exactly 5%. Simple calculation tells us that 35 ppt lowered by 5% would yield 33.25 ppt. To review; if I pour 5 gallons of RO/DI water to my system, my resulting salinity will be 33.25 ppt, or a SG of 1.024-1.025. My refractometer will 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. [/SIZE] So I add 5 gallons of RO/DI water to my system and wait a few hours to reach equillibrium. If I started out with 100 gallons of saltwater, then by dilution my new salinity should be 1.024-1.025. However, when I measure the salinity, I find instead that the new figure is 1.023, or 31 ppt. Instead of a 5% drop in salinity, I experience an 11% drop. My estimate was off by 6%. I now know that my total volume was not 100 gallons. It was 94 gallons. Since my salinity drop was greater than expected, I know my total volume was less than I thought. Had my salinity been off by 6% less than expected; I would know that my volume was more than my assumption. Make sense? At this point I could stop, and be satisfied that my total system volume is about 94 gallons. Alternatively I could also start over--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 must find out exactly how much flatwormeXit to add in order to deal with an outbreak of flatworms. If you decide that you'd like to lower your salinity by a point or two, knowledge instills confidence, and you can make the change without guesswork. 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 ensure the health of our livestock.