How Much Live Rock Do I Need? (A calculation)

by Dana Riddle

Here is a short encapsulation of Dana's method for calculating exactly how much live rock is required for your reef tank. Those of us who were lucky enough to have caught his live rock demonstration at Shark Bay in July got an in-depth explanation of this technique. For those of you who missed that meeting (or didn't bring your pad and paper), here it is again. This calculation is based on an active, running tank, however with certain assumptions, it can be used to predict how much rock to buy for a new tank. I am rewriting it based on notes taken during Dana's talk, and with repeated phone assistance from him during the actual typing of the article. Any mistakes are in my interpretation, and not his calculations.

In an existing tank, the first step is to test for the organic load. To do this, run a biochemical oxygen demand (BOD) test. You will need an accurate oxygen test kit for this.

1. Do a Dissolved Oxygen test on a 300 ml sample of aquarium water. Note the result. Place the stoppered sample in a cool, dark place (refrig) and 5 days later, do another DO test. Subtract this result from the original result. The difference is the BOD of your tank.
   

2. Next, calculate or measure your water turnover in the tank. In other words, how many times per day does the complete volume of water in your tank cycle through your filter system. DO NOT use the pump's GPH rating, as it stands. This is usually based on a flow rating measured with no head (vertical distance water must be pumped back into the aquarium), and is not accurate. Some pumps come with a chart which explains the maximum rate expected at a certain head. Obviously, the most accurate way to judge this is to measure the flow at the point where water reenters the tank. You will get a value (hopefully!) somewhere between 1-5 times the volume of the tank per hour. For example, you might find that your 600 GPH pump is actually only pumping 200 GPH through your tank. This is fine if you have a tank somewhere under 150 gallons or so.
   

3. Convert this number to millions of gallons per day by multiplying the GHP by 24, then dividing by 1,000,000. In our example above, the result would be 0.0048 million gallons per day. Multiply the result times your BOD (the number from your oxygen testing above), then multiply THAT times 8.34. The result will be the pounds of BOD that need to be removed each day from your tank.
   

4. Now that you know how much Biological Oxygen Demand (BOD) must be handled by the rock you can determine how much rock you'll need. Each cubic foot of rock can generally remove 0.11 lbs of BOD per day. Just divide the above number by 0.11 to determine how much rock is required. The result will be in cubic feet of rock.
   

5. For those of you who are getting ready to set up a new tank, a conservatively high BOD number you can use is around 2 mg per day. Most tanks will actually produce considerably less than this. If you expect your tank to have an exceptionally high bio-load (perhaps you plan to overstock, or keep animals known to generate lots of organic waste, such as sharks, lionfish, or other live-eaters), you can take this for granted.
   

6. You might ask (I did!), how the determination of BOD relates to the nitrification cycle, which most of us are used to using when defining bioloading. According to Dana's explanation, the BOD actually defines the oxygen requirements of a variety of processes within the aquarium, including the nitrication cycle itself. Thus, this number does relate to and encompass this value.
   

7. This calculation accurately determines how much rock your tank will NEED. The numbers generated from it are surprisingly small, especially in light of today's common assumptions of "2 pounds per gallon". The amount of rock required by your tank is considerably less than this. You may, of course, wish to have more simply for reasons of ascetics, but keep in mind, that that is what you're using
   it for. You are really not adding any filtration capacity by adding more than the calculated requirement.
   

David Ryder, President, AMAS

Reprinted form Reefscape, the newsletter of the Atlanta Marine Aquarijm Society To contact AMAS, call (770) 279-8310, or email to trigger@atlanta.com

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Reprinted with permission from AMAS. HTML'ized for RTO Features by Todd Zebert.