Making the calculations to determine how much of a supplement is required to adjust water chemistry.

Wed, 26 May 2010 19:58:42 -0400
General Tips

Supplements: All Ionic Supplements

One of my main goals when writing a label for a supplement is to provide sufficient information to enable the hobbyist to put said supplement to best use in their aquarium. With that in mind, there are generally four “main” sections of each label: an Overview (providing basic information on what the supplement has been formulated to accomplish), a Technical Background (a brief educational section that discusses the importance of what the supplement actually addresses), and two sets of instructions: Basic (for the casual hobbyist) and Advanced (for the more detail-oriented hobbyist). It is the Advanced Instructions, specifically as they pertain to all ionic (that is to say, inorganic) supplements, to which we will now turn our attention.
The purpose of the Advanced Instructions is to enable the hobbyist to perform some rudimentary calculations based on net water volume and the known concentration of the element that is being supplemented; without either of these pieces of information, making an accurate calculation to determine the required mass or volume of a supplement required to bring about a specific increase in concentration is impossible. So, in a nutshell, the first two things that must be known are the actual volume of water in the aquarium (this can be approximate, but the larger the system, the more important accuracy of this parameter becomes) and the concentration of the “target” element at the time of dosing. When determining the actual volume of water in the system, base your calculations on the inside dimensions and water depth of all aquaria, refugia, and sumps in the system; add the individual volumes together to get the total gross volume (you can also add the calculated volume of water in external filtration devices, plumbing, and so on, if you want to be more precise). Once you have done this, you will need to subtract the volume of water that is displaced by live rock, sand, and any other mass in the system. Lastly, an accurate test kit (with reagents that have not expired) should be used to determine the concentration of the target element in the aquarium system.

With the exception of Replenish and Koral Color (both of which are composed of multiple minor and trace elements), all Brightwell Aquatics ionic supplements list information on their labels pertaining to the increase in concentration of the target element that is attained when 1-ml of either a prepared solution or a stock solution is added to 1 US-gallon of water. The instructions then go on to describe how to determine the amount (in ppm) of the element that is required and how to calculate the corresponding dosage of the supplement. Performing these calculations can seem intimidating. Let’s face it: most people have blacked out their memories of high school or college chemistry classes, so it’s no surprise that this calculation might be seemingly overwhelming. Actually, it’s very quick and painless. For those of you who would like a refresher, follow the example below (we will use Magnesion in our example):

From the label: “Each ml of Magnesion will increase the concentration of magnesium (“[Mg2+]”) in 1 US-gallon (3.785 L) of water by 26 ppm. If the initial [Mg2+] in the aquarium is below 1,288 ppm, add this product at the maximum rate of 10 ml per 20 US-gallons daily until the desired concentration is attained, then dose daily or weekly as needed (see below). Always try to maintain the [Mg2+] within a range of +/-10 ppm. To determine the dosing rate of this product once the desired [Mg2+] has been acquired, measure the daily rate of magnesium uptake (i.e. the decrease in magnesium) in your aquarium by measuring the aquarium's [Mg2+] at the same time each day over a one- to two-week period. To determine the daily dosing rate (preferable to weekly dosing) for magnesium maintenance: estimate the volume of water in the entire aquarium system; divide the daily decrease in [Mg2+] by 26; multiply this number by the volume of water in the system to get the daily dosage required (ml) to maintain a stable [Mg2+].”

Now, let’s say that your aquarium holds100 US-gallons, the sump is filled to a depth that yields 25 US-gallons, and you have a refugium on the system that holds another 25 US-gallons. The gross water volume in the system is therefore 150 US-gallons.

100 US-gallons + 25 US-gallons + 25 US-gallons = 150 US-gallons

You have calculated the total water displacement from all of your rock and sand to be 50 US-gallons, and we will exclude the amount of water held in pipes for the purpose of simplification (if you want to include it in your calculations, add this volume to the net volume of the system). The net water volume is therefore 100 US-gallons.

150 US-gallons – 50 US-gallons = 100 US-gallons

You have determined that the current concentration of magnesium in your system is 1,100 ppm (shame on you). According to the label, the recommended magnesium concentration in reef aquaria is 1,288 ppm. Therefore, your system needs to undergo an increase of 188 ppm to reach the recommended concentration.

1,288 ppm – 1,100 ppm = 188 ppm

According the instructions, each milliliter of Magnesion will increase the concentration of magnesium in 1 US-gallon of water by ~26 ppm. Your system needs to come up by 188 ppm. In order to make this happen in 1 US-gallon of water, you would need to add 7.23 ml of Magnesion.

188 ppm ÷ 26 ppm = 7.23 ml per 1 US-gallon

You have 100 US-gallons in your aquarium, however. Therefore, you will need to add 72.3 ml of Magnesion to the system in order to bring about the increase from 1,100 ppm to 1,288 ppm.

7.23 ml per 1 US-gallon × 100 US-gallons = 723 ml

That’s all there is to it. Simply determine:
• Net water volume in the aquarium system
• The current concentration of the target element
• The required increase in ppm from the aquarium system to the recommended concentration (subtract “actual” from “recommended” concentration)

Then:
• Divide the required increase in ppm by the “ppm per ml per 1 US-gallon” number on the label
• Multiply your result by the net volume in the aquarium system

Once you know the net volume in the system, the most difficult part about performing the calculation is testing your water to determine the current concentration. After that, the calculation is over in less than 1-minute.

Now, for your homework this evening…