Using Salt to Determine Aquarium System Volume

aquarium salt

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Knowing your aquarium system volume is, quite literally, one of the most important things to know about your aquarium. System volume is important for determining species and numbers of animals an aquarium can hold, but even more importantly, volume determines how much of any individual product you’re going to dose from dechlor to complex ion preparations common in reefing or aquascape aquarium keeping.

For many of us who purchased a standard sized off-the-shelf aquarium, determining system volume is as easy as consulting the sales receipt. Unfortunately, it’s not always that easy. In some instances, people procure or build custom sized enclosures with unique shapes and footprints. In other instances, such as a paludariums for example, only a small portion of the aquarium volume is used for fish. To complicate matters further, some advanced aquarists have large multi-chamber filtration systems with extensive PVC plumbing and a bevy of devices that cumulatively hold a considerable percentage of the overall system volume. How does that aquarist determine their exact system volume?

The solution is surprisingly simple. For this project you will need the following:

All these items are essential components of an advanced aquarist’s equipment arsenal.

The procedure itself is relatively straightforward. You’re going to introduce a measured amount of salt into the tank, then compare salinity readings before and after the addition to determine the change in the system’s salinity. Utilizing this value, along with the known salt quantity, you can subsequently compute the water volume within the system.

Let’s go through this step by step. The first step is to provide an estimate of the system volume to use in step number two.

Step 1: Guesstimation

No, we’re not actually going to guess. We’re going to interpolate.

Simply take some measurements of your aquarium length, width, and height, you can measure in inches or centimeters, whichever is easier for you.

Then, multiply the length, width, and height of the aquarium in inches to get the volume in cubic inches.

Divide the cubic inch volume by 61 to convert it to liters, this value is a conversion factor between cubic inches and liters.

Volume in liters ≈ (Length × Width × Height) / 61

This will give you an approximate volume in liters based on the measurements in cubic inches. We’re using liters as our measure of volume as it’s used in the next step, but you can determine US gallons by dividing the same L x W x H in inches by 231 (the conversion factor between cubic inches and US gallons), and the formula will provide volume in gallons.

For those measuring in centimeters, the calculation to determine volume in liters is even easier, as the metric system is wont to do.

Multiply the length, width, and height of the aquarium in centimeters to get the volume in cubic centimeters (cm³).

Divide the cubic centimeter volume by 1000 to convert it to liters.

Volume in liters ≈ (Length × Width × Height) / 1000

Keep in mind that this is an approximation, and any large rocks, deep gravel beds, or even odd shapes of the aquarium might affect the accuracy of the calculation.

If you’re having trouble, try using this handy calculator to help you estimate your aquarium volume.

Okay, now that we have our volume estimated, let’s move on to step two.

Step 2: Weighing Your Options

The exact size of your addition largely depends on factors such as the aquarium’s size and type. For instance, if you’re managing a freshwater setup inhabited by fish, it’s wise to administer a modest salt dosage to prevent a substantial increase in salinity. Conversely, for systems housing saltwater fish at typical home aquarium salinity levels (specific gravity of 1.022 or salinity of 30g/L), you might opt for a more substantial increase, like introducing an entire 5 g/L increment to elevate salinity to the natural seawater level of 35 g/L. Larger changes in salinity will impart a more precise volume calculation.

A reasonable starting point for your addition is a salinity change of 1 g/L. In a general context, if you multiply your system’s volume (in liters) by this concentration value, the resulting mass signifies the number of grams of salt required to achieve that specific concentration.

For instance, you’re trying to increase the salinity of a 100-liter aquarium from zero to 1 g/L. In this scenario, 100 grams of salt would be necessary (1 g/L x 100 L = 100 g).

Once you’ve computed these values pertinent to your setup, measure out the designated mass of salt using a digital scale. Be prepared to introduce this measured salt in the next step.

Step 3: Salinity Adjustment

Before adding your salt, test and record your initial aquarium salinity. Once that’s done, it’s time to add the precise amount of salt you prepared in step 2 into the aquarium. After giving the salt some time to dissolve and mix in, go ahead and check the salinity of the aquarium once again. To find the change in salinity, simply subtract your initial salinity measurement from the final one.

SFinal – SInitial = ΔS

You can then calculate your entire system’s volume by factoring the number of grams of salt you added by the change in salinity.

This can be expressed by the following formula:

Volume = # grams of salt added / change in salinity

V = (salt added in grams) ÷ ΔS

Easy, right? Perhaps we had better look at an example.

Let’s say you have a small aquarium with hang-on filter that you want to determine an exact size for a salt treatment. First, using the method above, determine a rough volume. You measure the dimensions as 24” x 12” x 16” (L x W x H), using the conversion factor above you estimate your aquarium to have a volume of about 75 liters.

Using this approximated volume, you can estimate your salt requirement at about 225 grams. (75 liters x 3g/L = 225 grams)

Once added to your system, the salinity increases from an initial value of zero to a salinity value of 2.83 g/L, thus the change in salinity is 2.83 g/L.

Using the method outlined above, you’ll factor your salt added in grams by the change in salinity.

225 grams ÷ 2.83 g/L = 79.5 liters

Subsequently you can use this newly determined volume to put your salinity exactly where you intend.

In this instance, the goal was 3 g/L. In a similar way, using hte change in salinity necessary, one can determine exactly how much salt to add, provided a precise volume is known.

In this instance our change in salinity is 0.17g/L. (3g/L – 2.83g/L = 0.17g/L)

Multiplying this ΔS by our new volume calculation of 79.5 liters, we can determine that we’ll need about 13.5 grams of salt to reach our goal value. (0.17g/L x 79.5 liters = 13.515 grams)


The process of ascertaining the water volume in your aquarium has been simplified to the essentials: a salinity probe, a compact digital scale, and a calculator. Through the precise measurements of salinity changes and judicious salt additions, you gain the ability to pinpoint your aquarium’s water content with remarkable accuracy. This not only empowers you to tailor your care regimen to the unique needs of your aquatic environment but also underscores the significance of a holistic approach in maintaining a thriving aquatic ecosystem.


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