Aquarium Calculations

Chemical treatments can be ineffective if volume is underestimated and potentially lethal if it is overestimated. Before determining the concentration or amount of chemical to be used the water volume must first be calculated. Therefore, all fishkeepers should know the volume of their aquarium or pond.

Exact measurement of volume is essential in order to calculate any chemical applications and should be calculated before a problem occurs. Ponds preferably should be calculated when they are filled with water for the first time. The information is then recorded so it is immediately available when needed. In small ponds, depth should be measured across the pond in at least two directions. The number of different directions that will be needed will depend on the shape and bottom uniformity of the pond and will have to be determined on site. If water depth is not uniform it is important that average depth is measured. Greater number of depth measurements will result in greater accuracy.

Most aquariums used for holding fish are rectangular and the volume of rectangular aquariums is calculated by the formula: Volume (litres) = length × width × depth in centimetres ÷ 1000. When measuring a tank, take inside measurements of length and width and the depth at the appropriate water level. If the bottom of the tank is sloped, an average depth measurement should be used. To get the average depth of the tank, take three measurements: at the shallow end, in the middle and at the deep end. Add these depths together and divide the total by 3.

Circular pond or container volume is determined by the formula: Volume (litres) = 3.14 × radius² × depth in centimetres ÷ 1000. The radius is measured as ½ the inside diameter of the container. The radius is squared or multiplied by itself. For example, a circular container with an inside diameter of 180 cm and depth of 60 cm has a volume of 1526.04 litres (3.14 × 90 × × 60) ÷ 1000.

Aquarium Medication Calculations
All aquarium medications must be applied at a prescribed rate. Accurate application of this prescribed rate is necessary to achieve adequate control of the target organisms, and to avoid unwanted results such as mortality of nontarget organisms. Chemical application rates for aquariums are generally given as a final concentration of active ingredient in the water, usually in parts per million (ppm).

To calculate the dose rate of a chemical required in a given volume of water the formula is:
Dose rate = (required ppm × litres of water to be treated) ÷ percent of active ingredient.

[1 ppm = 1 mg/L; 1 million milligrams = 1 litre; 1000 milligrams = 1 gram; 1000 grams = 1 litre]

The easiest way to find out the total amount of chemical required is to convert the rates into something understandable like milligrams or grams. For example, to calculate the dose rate of 25 ppm (mg/L) of a chemical with an active ingredient of 400 grams/litre in 100 litres of water:

25 ppm x by total litres to be treated = 25 mg × 100 litres = 2500 mg. 2500 mg divided by percent (40%) of active ingredient = 2500 ÷ 0.40 = 6250 mg.

Example:
 = (25 ppm × 100 litres) ÷ 40%
 = (25 mg × 100 litres) ÷ 40%
 = 2500 mg ÷ 0.40
 = 6250 mg or 6.25 grams.

Useful Conversion Factors

© Copyright Adrian R. Tappin
Created July, 2005.