Understanding Water Hardness

In aquariums, water hardness is an important factor to consider for maintaining optimal conditions for fish and plants.

Water hardness is a measure of the concentration of certain minerals, primarily calcium and magnesium, in water. These minerals can form salts with anions such as bicarbonate, carbonate, and sulfate, which can affect the quality and properties of the water.


TDS, or Total Dissolved Solids, is a measure of all the inorganic and organic substances that are dissolved in the water. It includes minerals, salts, and other substances that can affect the water quality in an aquarium.

GH and KH

GH, or General Hardness, refers to the concentration of magnesium and calcium ions in the water, which contribute to the hardness of the water.

KH, or Carbonate Hardness, refers to the concentration of carbonate and bicarbonate ions in the water, which help to stabilize the pH of the water.

TDS, GH, and KH are related in that they all contribute to the overall water chemistry in an aquarium. While TDS includes all dissolved substances, GH and KH specifically measure the concentration of certain ions that can affect water hardness and pH stability.

Water hardness refers to the amount of calcium and magnesium cations present in a water source, which typically form salts with anions such as bicarbonate, carbonate, and sulfate.

Water hardness is generally categorized as either "hard" or "soft", with hard water containing high levels of calcium and magnesium ions and soft water containing low levels of these ions.

General hardness (GH) measures the concentration of calcium and magnesium cations, while carbonate hardness (KH) specifically refers to bicarbonate and carbonate anions.

However, the term "carbonate hardness" can be misleading because it actually measures the alkalinity of the water, or its ability to resist changes in pH due to acid additions. Other anions such as hydroxide, borates, silicates, and phosphates can also contribute to alkalinity, but the bicarbonate/carbonate buffering system is the primary source of alkalinity in plant aquariums.

GH and KH are usually similar in freshwater, with calcium and magnesium cations present in a 3:1 ratio and carbonates as the predominant anion. The levels of GH and KH are typically measured in degrees of general hardness (dGH) and carbonate hardness (dKH), respectively, with the latter expressed in ppm CO3 from CaCO3.


Conversion factors are used to convert between different units of measurement. For example, dGH can be converted to ppm CaCO3 by multiplying by 17.86, which was derived from the atomic weights of calcium and oxygen in CaO and CaCO3. Similarly, 1 dKH is equivalent to 17.86 ppm CaCO3 or 10.7145 ppm CO3.

These conversion factors can be useful for adjusting water chemistry in aquariums. For instance, adding 17.86 mg of CaCO3 can increase alkalinity by 1 dKH, while adding NaHCO3 can increase alkalinity by the same amount based on its molecular weight. However, it's important to note that different sources of anions and cations can affect GH and KH levels differently.

Overall, understanding water hardness and its units of measurement can help aquarists maintain optimal water conditions for their fish and plants.