The Fish Keeper’s Guide to pH, GH, and KH

pH, GH, and KH are terms commonly used in water chemistry, but there is a lot of confusion about them in the freshwater aquarium hobby. What is the difference between these parameters and how does it affect fish? This practical guide for beginners explains what these parameters mean, when you should test for them, and how to raise or lower their levels if needed.

pH (or Power of Hydrogen).

The pH of water measures the amount hydrogen ions present in it and can be used to determine how basic or acidic your water is. Pure water has a pH of 7.0 on a scale from 0-14. Acidic liquids (such as orange juice and vinegar) have a pH of less than 7.0, and alkaline liquids (like green tea and soap) have a pH of more than 7.0.

What pH level is ideal for aquariums?

Freshwater fish will tolerate pH levels of 6.5 to 8.0. South American fish and Caridina crystal shrimp tend to prefer lower pH, whereas African cichlids and livebearers prefer higher pH. Generally, the pH level isn’t a critical number to hit if you’re keeping fish for fun, but it can become more important if you’re trying to breed certain fish and raise their fry.

How to Measure pH

Aquarium Co-Op Multi-Test Strips include a test for measuring pH, and we recommend using it as part of your tank maintenance routine. If you have any questions about your fish’s health or want to keep them at a particular pH level, you can test their pH. If your aquarium has experienced a pH crash, your fish may show signs of stress, such as frantic swimming, lethargy, rapid breathing, or other erratic behavior.

The bottom line is that the pH of a fish tank changes naturally throughout the day. Most fish will adapt to a stable pH without sudden spikes.

AquariumCo-Op multi-test strips let you quickly and easily measure pH and KH in just one minute.

KH (or Carbonate Hardness)

KH measures the water’s level of carbonates or bicarbonates. This has an effect on the water’s buffering capacity. This means that KH helps neutralize acids and prevents your pH from changing too rapidly, which is useful because sudden pH crashes can cause health issues in your fish. Low KH indicates that your water is less buffered and can swing easily. High KH indicates that your water has a greater buffering capacity, and is more difficult to alter.

KH is like a trashcan. The trash can gets larger the higher KH. A pH crash can occur if the trash can is overflown. Therefore, people with low KH in their tap water often use crushed coral to gradually raise the KH (or increase the size of their trash can) and prevent pH crashes.

What is the Ideal KH Level For Aquariums?

KH can be measured in either dKH or ppm (parts/million), where 1 dKH equals 17.9ppm. Freshwater aquariums should typically have between 4-8 and 70-140 parts per million. You can lower the pH of animals such as crystal shrimp or discus by lowering the KH to 0-3dKH (or 50 ppm). African cichlids on the other hand prefer KH greater than 10 (or 180) ppm), which is often in harmony with higher pH levels.

How to Measure KH

Multi-test strips allow for easy measurement of KH. We use them as part our regular water changing routine. (Check out our guide to determine how often you need to be changing your water.) If you are trying to increase your KH level to avoid pH swings and b) if it is important to lower your KH, you might also need to measure KH.

Summary: Keep in mind that KH should not be below 2 dKH. This is because pH swings can easily occur and could potentially cause death for your animals. The exception to this rule is if your animals are sensitive to low pH. If your KH is very low, you can try these techniques to increase it.

GH (or General Hartness)

The water’s GH (calcium and magnesium ions) measures how hard or supple the water. It is one of many ways to check if your aquarium water has sufficient salts and minerals for healthy biological functions like fish muscle development, shrimp molting and snail shell formation, as well as plant growth.

What is the Ideal Level of GH for Aquariums?

As with KH, GH is measured in dGH (degrees of GH) and ppm. Freshwater aquariums should have a GH of between 4-8 dGH (or 70 to 140 ppm). While all animals require minerals, certain species of fish, such as livebearers, goldfish and African cichlids, need them more. If you’re trying to breed discus or other soft water fish, you may need to reduce the GH to 3 dGH (or 50 ppm) or below.

How to Measure GH

If you want to achieve a certain level of GH or if your plants and animals are suffering from health problems, we recommend the multi-test strips. Low GH can be manifested by:

Fish with low appetite, slow growth rate, and faded colors – Plants with signs calcium or other mineral deficiencies – Shrimp with trouble with molting – Snails that have thin, flaking, pitted shells

Remember that GH measures both calcium and magnesium, so if your water has high GH but you still see these symptoms, it’s possible your water has lots of magnesium but very little calcium. To determine if your water is deficient in this mineral, you can use a calcium test kit.

Bottom Line: Avoid lowering your GH values too much as this could lead to poor growth or even death for your animals and plants.

What is the relationship between pH, KH and GH?

The three ions that are measured in pH, KH and GH are specific types. The release of multiple types ions from a natural mineral source can have a significant impact on water parameters. Limestone, for example, contains a high amount of calcium carbonate. This contains both calcium and carbonate ions, and raises both GH (gravity) and KH (hydrogen). To increase GH, but not KH, increase the ions responsible for GH (calcium/magnesium) and exclude ions that can affect KH (carbonates/bicarbonates). Keepers of African cichlids often create or buy specific salt mixtures to raise KH and GH.

As mentioned before, KH directly relates to pH because it prevents your pH from changing as quickly. Aquariums have a tendency to lose pH over time. Therefore, if KH is increased, the pH value tends to remain higher. If you have an aquarium with a pH greater than 8.0 and add crushed coral as a buffering ingredient, KH will rise, but the pH value won’t change as much. However, if you have a lower pH and add crushed coral, both pH and KH values tend to increase.

How to Change pH, KH, and GH

There are many methods to reduce or raise the pH, KH and GH of your aquarium. Some are more effective than others, while some can be very dangerous. We prefer to be gentle and use more gentle methods. If you want to lower pH, KH, and GH and soften your water, we recommend letting the tank acidify over time by managing minimal water changes and gradually mixing in water filtered through an RODI (reverse osmosis de-ionized) water system.

If you wish to raise pH, KH, and GH and harden your water, our first choice is to add crushed coral – either mixed into the substrate or as a bag of filter media in your hang-on-back or canister filter. For fish health, our Washington retail store sells crushed coral. We recommend that you add 1 pound of crushed coral to every 10 gallons water. The lower your pH is, the faster it dissolves, so you may need to replace the crushed coral every 6 to 12 months to keep remineralizing your water.

Crushed coral

Wonder Shells or Seachem Equrium are another way to harden water. These supplements might not be needed if you have hard water from the tap. You may be able keep your mineral levels high by changing your water frequently.

Fish keepers, both novice and experienced, often take pH, KH and GH as a given. Don’t let this happen! Regular testing will help you catch many problems before they turn into full-blown disasters. This article was enjoyed by you. Sign up for our weekly newsletter to stay updated on the latest blog posts, videos and events.