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 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).

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. 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 and South American fish prefer a lower pH level, while African cichlids prefer a higher pH. The pH level of fish is not important if they are kept for enjoyment. However, it is more important if the fish are being raised to be breeders or to raise them.

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.

Bottom line: pH in a fish tank naturally changes throughout the day. The key is to maintain a relatively stable pH with no sudden spikes, and most fish will adapt.

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 amount of carbonates and bicarbonates in water, which affects the buffering capacity of the water. 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 means your water has more buffering capacity and the pH level is hard to change.

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 either in dKH, degrees of KH, or ppm (parts/million), where one dKH equals 17.9ppm. Typically, freshwater aquariums should be between 4-8 dKH (or 70-140 ppm). 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 find out how often to change 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 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 water’s GH (calcium and magnesium ions) measures how hard or supple the water. 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. 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

We recommend using the multi-test strips if you’re trying to reach a certain GH level or if your animals and plants are showing health issues. Low GH can be manifested by:

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

How are pH, KH, GH and GH related?

pH, KH, and GH all measure specific kinds of ions. The release of multiple types ions from a natural mineral source can have a significant impact on water 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. 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 value and add crushed coral, both KH and pH values tend to rise.

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 err on the side of caution by using gentler methods. You can lower pH, KH and GH, as well as soften your water by letting the tank get acidic. We suggest letting it do this slowly, with minimal water changes, and then gradually adding water filtered through an RODI water system.

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. You should replace crushed coral every six to twelve months to maintain a healthy pH.

Crushed coral

Wonder Shells or Seachem Equrium are another way to harden 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.

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.