The Fish Keeper’s Guide to PH, GH, And KH


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 are the differences between them and how can they affect our 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 or pH)

pH measures the amount of hydrogen ions in liquids and tells you how acidic or basic is your water. On a scale between 0 and 14, pure water has pH 7.0. Acidic liquids, such as vinegar and orange juice, have a pH lower than 7.0. Alkaline liquids like green tea and soap have a pH higher than 7.0.

What is the Ideal pH Level for Aquariums?

Freshwater fish will tolerate pH levels of 6.5 to 8.0. Caridina Crystal shrimp from South America and Caridina fishes prefer lower pH levels, while African cichlids like 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 contain a pH test. We recommend including it in your tank maintenance program. You may also want to test pH if your fish are having health problems or you need to maintain a certain pH level. Your fish could show signs of stress such as lethargy or rapid breathing, frantic swimming, and other unusual behavior if their pH has dropped.

Summary: The pH in a fish aquarium naturally changes throughout the day. Most fish will adapt to a stable pH without sudden spikes.

Aquarium co-Op multi test strips make it easy to measure pH, KH and GH in just a 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 means your water has less buffering capacity and the pH swings easily. High KH is a water with more buffering power and a higher pH. It’s difficult to alter the pH.

KH can be thought of as 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 is measured in dKH (degrees of KH) or ppm (parts per million), where 1 dKH equals 17.9 ppm. Freshwater aquariums should typically have between 4-8 and 70-140 parts per million. To lower the pH level for crystal shrimp and discus, you will need to reduce the KH from 0-3 dKH to 0-50ppm. 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

We like using the multi-test strips for easy measurement of KH as part of our regular water change routine. (Check out our guide to determine how often you need to be changing your water.) Other times you may want to measure KH include a) if you’re trying to raise your KH level to avoid pH swings or b) if you want to minimize your KH in order to lower your pH level.

Bottom line: In general, you don’t want KH to reach 2 dKH or below because then pH swings can easily happen and potentially kill your animals. You can raise certain animals that prefer low pH, but this is an exception. If your KH is very low, you can try these techniques to increase it.

GH (or General Hardness)

The GH is a measure of the amount calcium and magnesium ions present in the water. It also indicates how hard or soft the water is. 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. Ideally, freshwater aquariums have a GH between 4-8 dGH (or 70-140 ppm). All animals need some minerals, but certain fish like livebearers, goldfish, and African cichlids prefer higher GH levels. If you want to breed discus, or any other soft water fishes, the GH should be reduced to 3 dGH or 50 ppm.

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. Symptoms of low GH include:

– Fish with poor appetite, slow growth rate, lethargy, or faded colors – Plants with signs of calcium or other mineral deficiencies – Shrimp having trouble with molting – Snails with thin, flaking, or pitted shells

Remember that GH also measures calcium and magnesium. This means that if you have high GH but still experience these symptoms, your water may be low in magnesium, but high in calcium. To determine if your water is deficient in this mineral, you can use a calcium test kit.

Bottom line: Don’t let your GH values get too low because it may result in poor growth or even death with your animals and plants.

What is the relationship between pH, KH and GH?

All three of the ions measured are pH, KH, or GH. When minerals are added to water, they tend to release different types of ions. This can affect multiple parameters. Calcium carbonate is a good example. It contains both calcium ions and carbonate ions. Limestone has a high level of limestone, so it raises both GH & KH. 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.

KH, which is a preventative measure against rapid pH changes, directly correlates with pH. 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. If you have a lower pH, and add crushed coral to it, both the pH and KH values will increase.

How to change pH, KH, or GH

There are many ways to lower or increase the pH, KH, or GH in your aquarium. Some methods are less effective, others can be deadly. We prefer to be gentle and use more gentle methods. We recommend letting your tank become acidic over time. This can be done by using minimal water changes, slowly mixing in RODI (reverse osmosis-de-ionized water) filters, and allowing the tank to cool down.

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. To help our fish keep healthy, we use crushed coral in all of our tanks at the Washington retail store. 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

You can also use Wonder Shells and Seachem Equilibrium to harden your water. These supplements may not be required if your water is already hard. However, if your water is already hard, you might be able just to make extra water changes.

Both beginner and veteran fish keepers often take pH, KH, and GH for granted, so don’t fall into the trap of assuming those water parameters are always fine. Get into the habit of regularly testing for them as preventive maintenance, and you’ll catch a lot of problems before they become 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.