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. Pure water has a pH of 7.0 on a scale from 0-14. 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 pH level is ideal for aquariums?

The pH range of freshwater fish is between 6.5 and 8.0. Caridina shrimps from South America, and Caridina coral shrimps from Africa prefer pH levels between 6.5 and 8.0. African cichlids love higher pH levels. 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. You may also want to test pH if your fish are having health problems or you need to maintain a certain pH level. 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.

In short, the pH in a tank of fish changes over time. The key is to maintain a relatively stable pH with no sudden spikes, and most fish will adapt.

Aquarium CO-Op multitest strips enable you to quickly and efficiently measure pH, KH or GH in less than a minute.

KH (or Carbonate Hardness)

KH is a measure of water’s carbonates and bicarbonates. This affects water’s buffering ability. KH is used to neutralize acids and keep your pH from fluctuating too quickly. This is helpful because sudden pH changes can be dangerous for your fish’s health. Low KH can mean that your water has lower buffering capacity, and your pH swings often. High KH indicates that your water has a greater buffering capacity, and is more difficult to alter.

KH can be thought of as a trash can. The higher the KH, the larger the trash can. A pH crash will occur when that trash container is full. To prevent pH crashes, those with low KH tap waters often use crushed Coral to slowly raise it (or increase its size).

What is the ideal KH level for aquariums?

KH is measured as dKH (degrees or KH) and ppm, where 1dKH 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 however, prefer KH levels greater than 10 dKH or 180 ppm. This goes hand in hand to higher pH levels.

How to Measure KH

The multi-test strips are great for measuring KH. They can be used as part of our routine water changing. (Check out our guide to determine how often you need to be changing your water.) If you are trying to raise your pH level or to decrease your KH, you might also want to measure it.

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 to this rule is if your animals are sensitive to low pH. Try raising your KH to very low levels using the methods described below.

GH (or General Hartness)

The amount of calcium and magnesium in water (GH) is used to measure 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). Some animals need minerals. However, certain fish such as African cichlids (livebearers), goldfish, and goldfish require higher GH levels. If you plan to breed soft water fish such as discus, or any other species of fish, the GH should be reduced to at least 3 dGH (or 50ppm).

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 symptoms include:

Fish with a low appetite, slow growth rate or faded colours – Plants with signs or calcium deficiencies – Shrimp that has trouble molting – Snails with thin flaking shells or pitted nails

Keep in mind that GH is a measure of calcium and magnesium. If your water has a high GH, but you still have these symptoms, this could be because your water contains too much magnesium and not enough calcium. You can test your water with a calcium tester (specifically designed for fresh water), to see if it is lacking in that 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.

What is the relation between pH, KH and GH

The three ions that are measured in pH, KH and GH are specific types. When you add minerals to your water, it can release many types of ions which can then impact multiple 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. To increase GH, but not KH, increase only 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 is directly related to pH, as it prevents your pH level from rapidly changing. In aquariums, pH levels tend to drop over time, so when KH is raised, more acid is neutralized and pH tends to stay 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 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 use gentler methods. If you wish to reduce pH, KH or GH and soften water, we suggest letting the tank get acidic. Then, manage water changes slowly and mix water filtered through an RODI 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. Because Washington’s tap water is very soft, we use crushed Coral in every tank. This helps our fish stay healthy. It is recommended to add one pound of crushed Coral per 10 gallons. 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. 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.

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.