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).

pH is a measurement of the hydrogen ions found in liquids. It tells you how basic or acidic your water. 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 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. 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. 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. Your fish could show signs of stress such as lethargy or rapid breathing, frantic swimming, and other unusual behavior if their pH has dropped.

The bottom line is that the pH of a fish tank changes naturally 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 is a measure of water’s carbonates and bicarbonates. This affects water’s buffering ability. 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 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. 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 can be measured in either dKH or ppm (parts/million), where 1 dKH equals 17.9ppm. Freshwater aquariums should have a pH of between 4-8 dKH or 70-140ppm. To lower the pH level for crystal shrimp and discus, you will need to reduce the KH from 0-3 dKH to 0-50ppm. 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

Multi-test strips allow for easy measurement of KH. We use them as part our regular water changing routine. You can find our guide on how often you should change 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.) You can raise KH if you have low levels.

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. 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?

Similar to KH, GH can be measured in dGH or degrees of GH and ppm. Ideally, freshwater aquariums have a GH between 4-8 dGH (or 70-140 ppm). Although all animals need minerals, some fish, like African cichlids and livebearers require 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 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

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 but not enough calcium. If this happens, you should use a calcium testing kit (specifically for freshwater) to find out if you are lacking that particular mineral.

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, and GH Related?

All three of the ions measured are pH, KH, or GH. 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. To increase GH, but not KH, increase the ions responsible for GH (calcium/magnesium) and exclude ions that can affect KH (carbonates/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. The pH level of aquariums tends drop over time. If KH is raised, then more acid is neutralized, and pH tends 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. 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 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. 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

Another way to harden your water is by using Wonder Shells or Seachem Equilibrium. 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.

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. 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.