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    The calcium-magnesium ratio

    Nutrient optimization in the field of total hardness

    The elements calcium (Ca) and magnesium (Mg) and the other alkaline-earth metals (for example strontium and barium) jointly form the total hardness (GH) of water. A specific content of magnesium (4.3556 mg / l Mg) or calcium (7.144 mg / l Ca) in water make up one unit of the total hardness (1 ° dGH = one degree of total German hardness). The other alkaline earth metals are normally so poorly concentrated in the water that their contribution to the total hardness can be neglected.
    With suitable water tests the concentrations of magnesium and calcium as well as the total hardness of the water can be determined. Due to the above-mentioned relationships, one can always determine one of the three values by calculation, if one has the other two values available.
    This calculator is helpful. First select, from which element (magnesium or calcium) the concentration should be calculated. Then enter the known value of the total hardness (in ° dGH) in the Total Hardness (GH) field and the known concentration of the other element (in milligrams per liter) in the Reference Value field. Then click the button "Calculate". The computer program now determines the concentration of the missing component and also calculates the ratio of calcium to magnesium in the water.

    The ratio

    Both magnesium and calcium are nutrients for aquatic plants. Magnesium is similar to the important plant nutrient iron responsible for the formation of chlorophyll. It is consumed very little by the aquarium plants, but should be available in sufficient quantities (over 10 mg / l is recommended). In aquaristic practice, ratios of calcium to magnesium of about 3:1 to 4:1 have been found to be effective. A fairly modern approach is even an even lower ratio of only 2:1. Since many metal ions dissolved in the water compete for the nutrient uptake of aquatic plants, the macronutrient potassium (K) is added in the calculation. It is recommended that the content of magnesium should be higher than that of potassium. This is termed as the calcium/magnesium/potassium ratio, which ideally moves in the range of 2: 1: 0.5.

    Tap water

    If you use tap water for your aquarium, you should request an analysis of the water from your local drinking water supplier. Most of this information is on the websites of the respective drinking water suppliers. In addition to the total hardness, the concentrations for calcium and magnesium (in mg / l) are usually listed there. With help of the calculator you can determine the Ca:Mg-ratio of your tap water. Often there is already some magnesium present, but mostly insufficient, so that the Ca:Mg ratio is greater than 4:1. You can easily push the magnesium concentration by adding epsom salt (magnesium sulfate).

    epsom salt

    Water treatment

    If the contents of both elements are known in milligrams per liter (mg / l), one can now set the Ca: Mg ratio as desired. First, let's set our ideal Calcium:Magnesium Ratio (e.g., 3: 1). Based on the determined calcium content, we calculate the required magnesium concentration by the following formula:
    [Calcium content in mg/l] : 3 = [target magnesium content in mg/l]
    (Note: For a 4: 1 ratio, divide by 4 instead of 3. For a 2: 1 ratio, divide by 2 instead of 3.)
    If some magnesium is already contained in the starting water, we now subtract this amount from our target value:
    [Target magnesium content in mg/l] - [water magnesium content in mg/l] = [remaining difference]

    This difference needs to be pushed by adding epsom salt. In order to calculate this, we’ll use another nutrient calculator . We select the calculation method "Calculate dosage for specific nutrient content" and enter the number of liters of our aquarium under "Volume". In the nutrient category Magnesium, we choose the product epsom salt. The determined remaining difference in magnesium is entered in the “nutrient content” box. By clicking "Calculate" the program determines the amount of epsom salts to be added for our desired Ca:Mg ratio.

    Important notice: by adding epsom salt the aquarium’s total hardness is raised, too. The calculator will also show us how much the GH goes up.

    Since epsom salt is excellently soluble in water, the dry nutritional salt can just be added to the aquarium. For that, the portion gets spread evenly across the water surface.

    Water change

    Since magnesium is hardly consumed by the plants, it is usually sufficient to fertilize aquarium water once to the desired Ca:Mg ratio. Only with weekly water changes, the fresh water should be enriched with magnesium. The type of calculation is basically the same as described above. In this calculation, the sum of fed freshwater is entered into the nutrient calculator instead of the total volume of the aquarium.