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 is the difference between these parameters and how does it affect fish? This is a practical guide for beginners that explains how to determine what parameters are important, when they should be tested, and how to adjust them if needed.


pH (or Power of Hydrogen)

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 or vinegar have a pH less than 7.7, while alkaline liquids like green coffee and soap have a higher pH.

What is the ideal pH Level for Aquariums

Most freshwater fish can tolerate pH levels from 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 have a pH test. We recommend that you use it as part your tank maintenance routine. Other times you may want to test pH include a) when you are trying to maintain a specific pH level (such as for breeding purposes) or b) if you are troubleshooting health issues with your fish. 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.

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 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. KH neutralizes acids and prevents your pH level from rapidly changing. This is important because sudden pH drops can lead to health problems in fish. A low KH water level means that it has less buffering power and your pH swings more 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. The pH crash happens when the trash can gets too full. 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 have a pH of between 4-8 dKH or 70-140ppm. 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, 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 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.

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. You can raise certain animals that prefer low pH, but this is an exception. If you have very low KH, try to raise it using the techniques described below.

GH (or General Hartness)

GH measures the amount of calcium and magnesium ions in the water – in other words, how hard or soft your 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 GH Level 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

Multi-test strips are recommended if you want to increase your GH levels or if you have any health concerns with your pets and plants. 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?

The three ions that are measured in pH, KH and GH are specific types. 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. 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). African cichlid keepers frequently buy or make specific salt mixes to increase KH orGH.

KH is directly related to pH as it keeps your pH from fluctuating too quickly, as mentioned previously. In aquariums, pH levels tend to drop over time, so when KH is raised, more acid is neutralized and pH tends to 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, 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 use gentler 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. For fish health, our Washington retail store sells crushed coral. For every 10 gallons of water, you should add 1 pound to your substrate. To maintain your water’s mineralization, you will need to replace crushed coral every 6-12 months.

Crushed coral

You can also use Wonder Shells and Seachem Equilibrium to harden your water. 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.

Fish keepers, both novice and experienced, often take pH, KH and GH as a given. Don’t let this happen! You’ll be able to catch many problems before they escalate into major disasters if you get into the habit of testing them regularly as preventive maintenance. You’ll love this article! Don’t forget to subscribe to our weekly newsletter for the most recent blog posts, videos, events, and more!