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’s the difference between them, and how do they impact 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)

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?

Freshwater fish will tolerate pH levels of 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. While it’s not necessary to maintain a certain pH level if you keep fish for pleasure, it’s important to know if you want to breed fish or raise 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. 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. KH is useful for neutralizing acids and keeping your pH from changing too quickly. 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. KH is the size of the trash can. If we overflow that trash can, then a pH crash occurs. People with low KH tap water use crushed coral to slowly increase their KH (or increase their trash can’s size) 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. Freshwater aquariums should have a pH of between 4-8 dKH or 70-140ppm. If you need to lower the pH for animals like discus or crystal shrimp, you’ll need to decrease the KH to 0-3 dKH (or 0-50 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

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.) 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. The exception to this rule is if your animals are sensitive to low pH. You can raise KH if you have low levels.

GH (or General Hartness)

The water’s GH (calcium and magnesium ions) measures 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 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

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. 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 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.

What is the relationship between pH, KH and GH?

All three of the ions measured are pH, KH, or GH. When you add a natural source of minerals, it tends to release multiple types of ions, which then affects multiple types of water parameters. For example, limestone contains a high percentage of calcium carbonate, which contains both calcium and carbonate ions and therefore raises both GH and KH. You can only increase GH and not KH if you increase the specific ions of GH (calcium or magnesium), but you should not include ions that have an effect on KH (carbonates or bicarbonates). Keepers of African cichlids often create or buy specific salt mixtures to raise KH and GH.

As mentioned before, KH directly relates to pH because it prevents your pH from changing as quickly. 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, and GH

There are many, many ways to lower and raise the pH, KH, and GH in your aquarium – some that are less effective and others that can be dangerously potent. We prefer to err on the side of caution by using gentler methods. If you want to lower pH, KH, and GH and soften your water, we recommend letting the tank acidify over time by managing minimal water changes and gradually mixing in water filtered through an RODI (reverse osmosis de-ionized) 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. When adding it to your substrate, we recommend starting with 1 pound of crushed coral per 10 gallons of 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

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.

Fish keepers, both novice and experienced, often take pH, KH and GH as a given. Don’t let this happen! 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. You’ll love this article! Don’t forget to subscribe to our weekly newsletter for the most recent blog posts, videos, events, and more!