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

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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 guide is for beginners and explains the meaning of these parameters, how to test for them, and how you can raise or lower them if necessary.


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 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 is the ideal pH Level for Aquariums

Most freshwater fish can tolerate pH levels from 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. 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. It is important to keep the pH stable and not spike suddenly. Fish will adjust.

Aquarium Co-Op multi-test strips allow you to quickly and easily measure pH, KH, and GH in just 1 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 indicates that your water has a greater buffering capacity, and is more difficult to alter.

KH can be thought of as a trashcan. KH is the size of the trash can. If we overflow that trash can, then a pH crash occurs. Low KH tap water users often use crushed coral as a way to slowly raise it (or increase the trash can) and avoid 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. 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, appreciate KH higher than 10 dKH (or 180 ppm), which usually goes hand in hand 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 find out how often to change your water. You may also want to measure your KH if your goal is to increase your KH level in order to prevent pH swings, or b) to reduce your KH to lower your pH.

The bottom line: You don’t want your KH to drop below 2 dKH because that can cause pH swings and possibly kill your animals. (The exception is if you’re raising certain animals that like low pH.) 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. This is one of the most effective ways to determine if your aquarium water contains enough minerals and salts that are necessary for healthy biological functions such as shrimp molting, fish muscle and bone development, and snail shell development.

What is the ideal GH level for aquariums?

As with KH, GH is measured in dGH (degrees of GH) and ppm. Freshwater aquariums should have an ideal GH between 4-8 and 70-140 ppm. Although all animals need minerals, some fish, like African cichlids and livebearers require higher GH levels. You may need to lower the GH level to 3 dGH (or 50ppm) if you are trying to breed discus and other soft-water fish.

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 that are unable to eat, have slow growth rates, or show signs of lethargy – Plants showing signs of calcium or another mineral deficiencies – Shrimps having difficulty with molting – Shells on snails that are thin, flaking or pitted

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. If this happens, you should use a calcium testing kit (specifically for freshwater) to find out if you are lacking that particular mineral.

The bottom line: Do not let your GH values drop too low as it could cause poor growth, or even death in your plants and animals.

How are pH, KH, GH and GH related?

pH, KH, and GH all measure specific kinds of ions. When you add a natural source of minerals, it tends to release multiple types of ions, which then affects multiple types of 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). If you want to increase only GH but not KH, you must increase the specific ions for GH (calcium and magnesium) without including ions that affect KH (carbonates and bicarbonates). Keepers of African cichlids often create or buy specific salt mixtures to raise KH and GH.

KH is directly related to pH as it keeps your pH from fluctuating too quickly, as mentioned previously. The pH level of aquariums tends drop over time. If KH is raised, then more acid is neutralized, and pH tends stay higher. We have observed that when you have a pH higher than 8.0, and add a buffering agent such as crushed coral, KH will increase 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. 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.

Crushed coral can be used to increase pH, KH and GH, or to filter your water. It can be mixed in to the substrate, or used as a bag of media in your hang-on back or canister filters. Our retail store in Washington has very soft tap water, so we use crushed coral in every tank to help our fish stay healthy. For every 10 gallons of water, you should add 1 pound to your substrate. You should replace crushed coral every six to twelve months to maintain a healthy pH.

Crushed coral

Another way to harden your water is by using Wonder Shells or Seachem Equilibrium. If you already have hard water coming out of the tap, these supplements may not be necessary, and you may be able to keep the mineral levels high just by doing extra water changes.

Beginner and veteran fish keepers alike often take pH and KH for granted. However, don’t allow yourself to fall into the trap and assume that these water parameters are always perfect. Regular testing will help you catch many problems before they turn into full-blown disasters. You’ll love this article! Don’t forget to subscribe to our weekly newsletter for the most recent blog posts, videos, events, and more!