Understanding Water Hardness in Aquariums: General and Carbonate Hardness Explained

Understanding Water Chemistry

Water chemistry is a topic that often confuses many aquarists. While most hobbyists are familiar with the importance of monitoring ammonia, nitrites, and nitrates, understanding terms like pH, KH, and GH can be a bit tricky. This article aims to explain these concepts clearly and help you get a better grasp on your tank’s water parameters.

What is General Hardness?

General Hardness (GH), also known as Total Hardness, measures the concentration of dissolved minerals in water.

The hardness of water depends on the type of land it flows through before reaching its final destination. Water that passes over rocky regions tends to have a higher mineral content, while water flowing through marshy or forested areas usually results in softer water, commonly found in rivers or lakes.

For example, fish like Neon tetras and Cardinal tetras thrive in soft, acidic water.

Tap water typically contains various minerals absorbed along its journey, primarily magnesium and calcium, along with trace amounts of other metals such as iron and zinc. The concentration of these minerals is what determines the general hardness of the water.

When selecting fish for your aquarium, the recommended water conditions usually include GH, pH, and temperature. While South American cichlids prefer soft water, fish from Lake Malawi thrive in very hard water. GH test kits can provide results in either DH (degrees of hardness) or PPM (parts per million). Luckily, converting between these two scales is simple using the right formula.

Water Hardness Levels

These numbers might vary slightly depending on the reference. To convert DH to PPM, multiply the DH by 17.9. To convert PPM to DH, multiply the PPM by 0.056.

If you want to increase the GH in your water, you can add calcium-based rocks like limestone or coral gravel. These will slowly dissolve and raise the mineral content over time. Some aquarists prefer using calcium-based powders for quicker results, but the gradual method is often more stable.

Although most fish can adjust to different GH levels, breeders need to maintain the correct GH in their tanks as it can significantly affect egg fertility and hatch rates.

Understanding Carbonate Hardness (KH)

Carbonate hardness (KH) refers to the water’s buffering capacity, or how much acid can be added before the pH level is affected.

In an aquarium, nitrates naturally produce nitric acid, which can lower the pH if the water doesn’t have enough buffering capacity to neutralize it. It’s generally recommended that the KH level stays above 4.5 degrees to maintain pH stability, as pH fluctuations can be harmful to fish.

This is why regular water changes are vital in fishkeeping. By lowering nitrate levels, you reduce the amount of nitric acid produced, helping to maintain a more stable pH over time. The higher the carbonate hardness, the better the water's ability to buffer pH changes.

In simple terms, KH measures the concentration of bicarbonate and carbonate ions in the water. Carbonate ions bond with free hydrogen ions, and as more hydrogen ions are bonded, the pH rises. Free hydrogen ions, which acidify the water, lower the pH. When carbonate ions bond with hydrogen, they form carbonic acid (H₂CO₃), which eventually breaks down into carbon dioxide (CO₂) and escapes into the atmosphere.

Do Plants Affect Carbonate Hardness?

Plants in an aquarium can reduce carbonate hardness, but the effect depends on the conditions in the tank. In low-CO₂ environments, some aquatic plants can utilize bicarbonate (HCO₃⁻) as a carbon source for photosynthesis. When plants take up bicarbonates, they indirectly reduce carbonate hardness. This process can lead to a gradual decrease in KH over time.

However, the impact on KH might be limited in tanks with sufficient CO₂ supplementation. In such environments, plants prefer using dissolved CO₂ rather than bicarbonates, which helps maintain more stable KH levels. Additionally, other factors, such as water changes, substrate buffering, and the presence of specific rocks, play a significant role in influencing KH. So, while plants can affect KH, they are not always the main factor in its fluctuation.

Managing KH in Your Aquarium

If the GH (General Hardness) or KH levels are too high for your fish, you can reduce them using various methods. For example, reverse osmosis (RO) water removes many carbonate ions and minerals, reducing water hardness. This also lowers pH, making it more acidic, which worked well for my Discus tanks, where a pH of 6 to 6.5 is ideal. However, for my marine setup, I now remineralize the RO water, as it needs a pH of at least 8.2. Without sufficient KH, RO water can become unstable, as it loses its buffering capacity. Adding salt helps restore the KH/pH balance in marine tanks.

If your KH is too low, you can increase it using sodium bicarbonate. I found that adding 5 ml of sodium bicarbonate to every 40 UK gallons (approximately 33.4 US gallons or 151.4 liters) of water raised the KH by 4 degrees without significantly affecting the pH, but it’s important to monitor the levels closely.

Adding oxygen to the water can also help drive off CO₂, improving the buffering capacity. You can also find commercial buffering products, but be cautious, as some may increase phosphate levels in the water.

As you can see, GH, KH, and pH are interconnected, and fluctuations in one can impact the others. The best approach is to choose fish that match your water conditions rather than constantly adjusting the water to fit the fish. Over time, adjusted water tends to return to its original state.