Understanding Carbonate Hardness (KH) is fundamental for maintaining stable water conditions, especially in closed systems like aquariums. KH is commonly referred to as alkalinity, which measures the water’s capacity to resist changes in its pH level. This resistance to pH fluctuation is crucial for the health and stability of aquatic environments. Managing KH correctly prevents chemical imbalances that can affect organisms.
Defining Carbonate Hardness
Carbonate Hardness (KH) chemically defines the concentration of carbonate (\(\text{CO}_3^{2-}\)) and bicarbonate (\(\text{HCO}_3^-\)) ions dissolved in the water. These ions originate primarily from dissolved mineral salts like calcium carbonate and magnesium carbonate. The presence of these compounds makes up the portion of total water hardness known as temporary hardness. This term is used because these compounds can precipitate out of the water, such as the limescale left after boiling.
KH is often measured in units called degrees of Carbonate Hardness (dKH), which comes from the German term Karbonathärte. Another common way to express this measurement is in parts per million (ppm) of calcium carbonate (\(\text{CaCO}_3\)) equivalents. One dKH is approximately equivalent to 17.85 ppm of calcium carbonate equivalents. This value indicates the amount of buffering capability present in the water.
KH’s Essential Role in pH Stability
The primary function of Carbonate Hardness is to act as a chemical buffer, preventing drastic shifts in the water’s pH. This buffering action occurs because the bicarbonate and carbonate ions readily neutralize acids constantly introduced into the water. In a closed system, acids are naturally produced through biological processes, such as fish waste and the nitrification cycle carried out by beneficial bacteria.
When an acid enters the water, the carbonate and bicarbonate ions absorb the free hydrogen ions (\(H^+\)). By absorbing these ions, the buffer system prevents them from significantly altering the water’s overall acidity. This action is crucial because a sudden drop in pH, often called a “pH crash,” can be highly stressful or fatal to aquatic organisms.
A low KH level means the water has poor buffering capacity, making it vulnerable to rapid pH changes. Conversely, a higher KH level provides a stronger, more resilient buffer, making the pH much more stable. Maintaining a stable pH, rather than chasing a specific number, is the most important factor, and KH ensures this stability. For most freshwater environments, a KH of 4 to 8 dKH (approximately 70 to 140 ppm) is considered favorable.
Distinguishing KH from General Hardness
KH is frequently confused with General Hardness (GH), but they measure two distinct properties of the water. KH measures the concentration of carbonate and bicarbonate ions that serve as the pH buffer. GH measures the total concentration of dissolved multivalent metallic cations, primarily calcium (\(\text{Ca}^{2+}\)) and magnesium (\(\text{Mg}^{2+}\)) ions.
GH is important for biological processes, such as osmoregulation in fish and the healthy growth of shells and plant tissues. These ions do not significantly contribute to the water’s buffering capacity. KH is often referred to as temporary hardness because the ions can precipitate out, whereas GH, particularly the part not bound to carbonates, is considered permanent hardness. It is possible to have high GH and low KH, or vice versa, demonstrating that the two measurements are independent.
Practical Steps for Adjusting KH
Adjusting the Carbonate Hardness level is a controlled process that must begin with accurate testing of the current water parameters. To raise KH, the most common method is the controlled addition of sodium bicarbonate (baking soda). Commercial buffer products containing carbonate salts are also available to increase the buffering capacity. Crushed coral or aragonite substrate can slowly dissolve over time, releasing calcium carbonate and thereby raising both KH and GH.
Lowering the KH level is achieved most effectively through dilution with water that has a near-zero mineral content. This includes using distilled water or water purified by a reverse osmosis (RO) system. These pure water sources lack the carbonate and bicarbonate ions, and mixing them with the existing water will proportionally decrease the KH. All adjustments should be made slowly over a period of days to avoid shocking organisms, and the KH must be re-tested after each adjustment to confirm the desired level.