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 practical guide for beginners explains what these parameters mean, when you should test for them, and how to raise or lower their levels 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 of 0 to 14, pure water has a neutral pH of 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 are happy at pH levels between 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. 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.

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 allow you to quickly and easily measure pH, KH, and GH in just 1 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 means your water has less buffering capacity and the pH swings easily. High KH means your water has more buffering capacity and the pH level is hard to change.

Think of KH like a trash can. The higher the KH, the larger the trash can. If we overflow that trash can, then a pH crash occurs. 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. Typically, freshwater aquariums should be between 4-8 dKH (or 70-140 ppm). 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, 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.  

Bottom line: In general, you don’t want KH to reach 2 dKH or below because then pH swings can easily happen and potentially kill your animals. (The exception is if you’re raising certain animals that like low pH.) If you have very low KH, try to raise it using the techniques described below.

GH (or General Hardness)

GH measures the amount of calcium and magnesium ions in the water – in other words, how hard or soft your water is. It’s one of the easiest ways to measure if your aquarium water has enough salts and minerals that are essential for healthy biological functions, such as fish muscle and bone development, shrimp molting, snail shell development, and 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’re trying to breed discus or other soft water fish, you may need to reduce the GH to 3 dGH (or 50 ppm) or below.

GH hardness chart for aquariums

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. In this case, use a calcium test kit (specifically for fresh water) to determine if you’re lacking that particular mineral. 

Bottom line: Don’t let your GH values get too low because it may result in poor growth or even death with your animals and plants.

How are pH, KH, 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. For example, limestone contains a high percentage of calcium carbonate, which contains both calcium and carbonate ions and therefore raises both GH and 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). In fact, African cichlid keepers often buy or create specific salt mixes to individually raise KH or 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. For instance, we’ve observed that if you have a higher pH of 8.0 and you add a buffering agent like crushed coral, KH will rise but the pH value doesn’t move 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.

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. Our retail store in Washington has very soft tap water, so we use crushed coral in every tank to help our fish stay healthy. 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

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.

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. If you enjoyed this article, don’t forget to sign up for our weekly newsletter so you won’t miss our latest blog posts, videos, and events!