What Are GH and KH In An Aquarium?

General hardness (GH) measures the amount of dissolved calcium and magnesium in your aquarium water. These minerals are important for the healthy growth of fish, snails, shrimp, and aquatic plants. GH also helps you know which fish species are best suited to your aquarium. 

Carbonate hardness (KH) measures the amount of carbonate and bicarbonate in the water. Its main job is to maintain a stable pH by preventing sudden changes. These are known as pH crashes, which can be quite harmful to fish. Even though GH and KH are both related to water hardness, they measure different things. Testing both can give you a better understanding of your aquarium's water quality. 

For most community aquariums, a GH of 4-12 dGH and a KH of 3-8 dKH works well. However, the right level depends on the fish and plants you keep, as different species have different water needs. 

Check out Why Regular Aquarium Water Testing Is Crucial for Your Fish’s Health. 

What Is General Hardness?

General Hardness (GH) measures the amount of dissolved calcium and magnesium in your aquarium water. It tells you if the water is hard or soft. GH is an important indicator that shows if your aquarium contains the minerals needed for healthy aquatic life. These minerals support essential biological processes. It includes: 

• Successful molting in shrimp

• Strong bones and muscle development in fish. 

• Healthy shell growth in snails

• Proper growth and nutrient uptake in aquatic plants. 

Maintaining the right GH levels helps create a healthy environment for your fish and plants to grow.

What Is the Ideal GH Level for an Aquarium?

General Hardness (GH) is measured in degrees of GH (dGH) or parts per million (ppm). For most freshwater aquariums, the ideal GH range is 4-8 dGH (70-140 ppm). It provides enough calcium and magnesium to support the health of fish, shrimp, snails, and even aquatic plants. However, the best GH levels depend on the species you keep. 

• Livebearers (such as mollies, guppies, and platies), goldfish, and African cichlids prefer harder water with higher GH. 

• Discus and most other soft-water fish can grow in lower GH levels. If you are breeding these species, you need to reduce the GH to 3 dGH (50 ppm) or even lower. 

However, you need to do the research for the water requirements of your fish and plants. When you maintain the correct GH, it helps keep them healthy and even reduces stress.

How To Measure GH?

The easiest way to measure General Hardness (GH) is with the use of a GH test kit, or you can use multi-parameter aquarium test strips. Regular testing can help you maintain the right mineral levels for your fish, snails, shrimp, and plants. It is always a good idea to test your GH if you are trying to reach a specific water hardness level or if you notice any signs of mineral deficiencies, such as: 

• Fish that have a poor appetite, appear inactive, grow slowly, or lose their bright colors. 

• Aquatic plants that show poor growth or signs of calcium and mineral deficiencies.

• Shrimp are struggling to mold properly. 

• Snails with cracked, thin, flaky, or pitted shells. 

However, keep in mind that GH can measure both calcium and magnesium. If your GH reading is high but you still notice these issues, your water can contain a lot of magnesium but not enough calcium. In that case, you need to use a freshwater calcium test kit that checks your calcium levels. It can help find out the problem and make the needed adjustments to keep your aquarium healthy.

What Does Carbonate Hardness Measure?

Carbonate Hardness (KH) measures the amount of carbonate and bicarbonate in your aquarium water. These minerals work as a buffer that helps keep the PH stable. KH is also known as alkalinity or buffering capacity and is measured in degrees of KH (dKH) or parts per million (ppm). 

KH doesn't directly affect the health of the fish or plants. Instead, its main role is to prevent sudden changes in the pH levels. Acids are constantly produced in an aquarium through fish waste, carbon dioxide (CO2), and the nitrogen cycle, which produces nitric acid. These acids can naturally lower the pH over time. KH neutralizes these acids, which also helps maintain a stable pH.

When KH is at a healthy level, your aquarium's pH remains stable. However, if KH drops too low or reaches zero, it can no longer buffer the water. It can cause a sudden and dangerous drop in pH, known as a pH crash or old tank syndrome, that can stress or even kill fish and other aquatic life. It is also important to maintain an appropriate KH level to keep your aquarium's water chemistry stable and your fish healthy.

What Is The Ideal KH Level For An Aquarium?

Carbonate Hardness (KH) is measured in degrees of KH (dKH) or parts per million (ppm), where 1 dKH equals 17.9 ppm. For most freshwater aquariums, the ideal KH range is 4-8 dKH (70-140 ppm). It provides enough buffering capacity to keep the PH stable and reduces the risk of sudden pH changes. However, the ideal KH depends on the species you keep. 

• Discus, crystal shrimp, and other soft-water species usually do their best with a KH of 0-3 dKH (0-50 ppm). It helps maintain a lower pH. 

• African cichlids prefer hard, alkaline water and usually grow well with a KH above 10 dKH (180 ppm. 

Why KH Matters?

In most aquariums, KH should not fall below 2 dKH. If the levels are very low, the water can lose its ability to resist changes in pH. As a result, there can be sudden pH crashes. These quick changes can stress or even kill fish, shrimp, and other aquatic species. 

The main exception is aquariums that keep species that need naturally soft, acidic water. Even then, KH and pH should be checked carefully to keep conditions safe and stable.

Learn why Aquarium pH Crashes: What Causes Them and How to Fix Them Fast.

How To Safely Adjust KH and GH in The Aquarium?

There are several ways to raise or lower KH and GH in an aquarium. Some methods work safely, but others can change water chemistry too quickly and even stress your fish. In most cases, slow and steady adjustments are the right approach. 

If you want softer, more acidic water, you can mix RO or deionized water with your tap water to reduce mineral levels. Make sure to let the aquarium naturally become slightly acidic over time by avoiding unnecessary larger changes. You can even use natural minerals like Indian almond leaves or peat moss. It can soften the water and lower the pH. 

However, when you need to increase water hardness and stabilize pH, use crushed coral. As it slowly dissolves, it releases calcium carbonate into the water, raising GH and KH over time. Crushed corals can be mixed into the aquarium substrate. It should be placed inside a mesh bag in a hang-on-back or canister filter. A common starting guideline is about 1 pound (450 g) of crushed coral per 10 gallons (38 liters) of aquarium water.