View Full Version : PH, KH And GH in Depth

06-14-2013, 09:27 AM
There are questions about pH, KH, and GH posted on this forum nearly every day. In this thread, I will discuss what these mean and how they affect your pond. Please do not post comments directly in this thread. If you have any questions or comments, you may PM me or start a new thread and we can have further discussions there.

For those who would like to print out this thread, the complete text is in the attached PDF file. Because of the limitations of the buletin board software, I will have to break the document up into multiple posts, and I do not know how long that will take, so please be patient while I add the posts. I hope you find the discussion helpful.


06-14-2013, 09:39 AM
What do pH, KH, and GH really mean, and how they affect your pond

We all know that water is H2O; however, in nature, some of the water molecules separate into hydrogen ions (H+) and hydroxide ions (OH-). In pure water, the number of H+ ions and the number of OH- ions are equal. Pure water never exists in nature, because water is an excellent solvent. Even if you have bottled distilled water, as soon as the water was put in the bottle, trace quantities of the material that makes up the container will dissolve into the water. Once you open the bottle, gases from the atmosphere will also dissolve into the water, and once you pour the water into another container, the water will pick up traces of almost anything with which it comes into contact. Carbon dioxide, ammonia, or any acid or base that is added to water will affect the ratio of H+ to OH in the solution.


pH is a unit of measure of the concentration of H+ ions in solution. Specifically, pH is defined as the decimal logarithm of the reciprocal of the hydrogen ion activity, aH+, in a solution.

06-14-2013, 09:45 AM
Similarily, pOH is the decimal logarithm of the reciprocal of the hydroxide ion activity, aOH-, in a solution. One of the laws of nature is that there is a fixed relationship between pH and pOH such that pH + pOH always add up to 14. Thus, if you know the pH, you also know the pOH. The pH of pure water at 25°C is 7. Since pH and pOH are proportional to the reciprocal of the concentration, and since the sum of pH and pOH is always a constant value, whenever H+ is added to a solution or OH is removed from the solution, the pH will go down. Conversely, whenever OH- is added to a solution or H+ is removed from the solution, pH will go up. On a log scale, the relationship between H+ and OH concentrations and pH are linear as shown in Figure 1.

Figure 1: The relationship between H+ and OH- concentrations on a log scale

06-14-2013, 09:49 AM
Figure 1 is shown on a log scale, where each increment on the Y-axis is ten times more than the previous increment. This is important to remember when thinking about how pH relates to the actual H+ (or OH-) concentrations. A drop in pH from 8 to 7 is equal, with respect to the change in H+ concentration, to a drop in pH from 7 to 6.72, and the change from pH 7 to 6 is a ten times larger change with respect to the actual H+ concentration than a change from pH 7 to 8 is.
The relationship between H+ and OH- concentrations and their effects on pH are shown on a Cartesian axis in Figure 2

Figure 2: The relationship between H+ and HO- concentrations on a Cartesian scale

06-14-2013, 09:52 AM
Because the concentrations are so high at the extremes of pH, it is not really possible to see the changes in concentration in the middle. For the purposes of our ponds, we are really only concerned about the pH range of 5 to 9. The relationship between H+ and OH- concentrations in the “practical pond range” are shown in Figure 3

Figure 3: The relationship between H+ and OH- concentrations within the pH range that would be found in a pond setting

06-14-2013, 09:54 AM
While the difference in H+ concentration between pH 6 and 7 is still a factor of 10, the absolute change in concentration is not nearly as great as the change from 5 to 6 would be. When we speak of “pH crash”, what is really dangerous is when pH goes below 6.5. A drop from 7.5 to 6.5 is not nearly as hard on the fish or the biofilter as a drop from 7 to 6 would be, even though both are a one unit change in pH.

1.1. What is the ideal pH for koi?

Many koi kichi will say that the best beni is produced by koi that are raised in soft water with a pH in the 6.8 to 7.2 range. While that might be true, koi are adaptable to a wide range of pH from about 6.4 to 9.4 as long as the pH is stable. For reasons that I will discuss under the section on KH, the vast majority of koi keepers have a stable pH of 8.3. Would their koi look better if the pH was 7.0? Perhaps, but they would not necessarily be any healthier. For healthy, long-lived koi, the most important thing about pH is that it be stable.

1.2. What should I do if my pH is too high or too low?

It is rarely a good idea to try to adjust the pH directly. Adding an acid, such as acetic acid (vinegar), to bring the pH down or a base, such as sodium hydroxide (NaOH), to bring the pH up will be either risky or futile, depending on what the KH of the water is. If the KH is low, addition of even a small quantity of acid or base can cause very large changes in pH, which is not healthy for the koi. If the KH is high, then adding an acid or base will have no lasting effect on the pH. It is best to maintain the KH, as I will discuss below, and the pH will take care of itself.
There is one exception to what I have just said about pH. If your source water has a very low pH, and especially if you are using well water, there is a good chance that the water has a very high carbon dioxide content. Carbon dioxide can suffocate fish, even if the oxygen concentration is near the saturation point. To prevent the carbon dioxide from entering the pond, I recommend putting an aerator that was designed for a laundry sink on the end of the hose. This will remove the carbon dioxide and raise the pH as the water is being added to the pond. You will still need to check the KH to maintain a stable pH, but the aerator will help.

06-14-2013, 10:10 AM

KH is the abbreviation for “Karbonat härte” (more correctly written as a compound word, “Karbonathärte”), which is German for “carbonate hardness”. Do not let the name fool you. Carbonate hardness (also known as temporary hardness) really is the term for the resistance of a solution to a change in pH. The resistance to change in pH can also be referred to as “buffer capacity” or “alkalinity”.

2.1. What is a buffer?

A buffer is a compound in solution that will either pick up or give up a H+ in response to addition of H+ or OH- to the solution so that the concentration of free H+ remains constant. Buffers are formed by mixing a weak acid and its salt. Although carbonate hardness is standardized in terms of how much carbonate (CO3--) would need to be present to account for the resistance to change in pH that was found in a solution, there is no requirement for any carbonate to be present. Any combination of a weak acid and its salt forms a buffer, and all buffers contribute to KH. Since we do not know which buffer is present, and since each buffer has a different molecular weight, carbonate was chosen to be the standard against which all buffers are measured. KH can be expressed in German degrees of hardness (°dH) or parts per million (ppm). One °dH is equal to 17.85 ppm.

While KH tells you how easy or difficult it will be for the pH to change, unless you know what buffer is present in the solution, the KH does not tell you anything about what the pH will be. Any buffer will increase the KH. The higher the KH, the more difficult it is to change the pH, but each buffer has its own stable pH point.

Pure water has a KH of zero (0). As stated above, when H+ is added [e.g., by adding an acid like hydrochloric acid (HCl) or sulfuric acid (H2SO4)] to pure water, the pH automatically goes down, and when OH- is added [e.g., by adding a base like sodium hydroxide (NaOH)], or when an H+ is removed [as happens when ammonia (NH3) is added to water and picks up a H+ to form ammonium (NH4+)], the pH goes up. There is no resistance to the change in pH.

2.2. How does KH affect the pond?

The life forms in a pond, whether they are fish, bacteria, algae, or higher plants, all produce waste that tend to lower pH. Many also produce ammonia, which will raise pH; however, except in a new pond that has not fully cycled, the forces trying to bring pH down almost always far exceed the forces that try to push pH up. In a pond with low KH (< 80 ppm), there is a risk that pH will change suddenly. If your source water has a high KH, then regular water changes are often enough to maintain an adequate KH in the pond. The pH of the pond water will be whatever the pH of the source water is. On the other hand, if the KH of the source water is low, a buffer should be added to the pond water to increase the KH in the pond, and the pH of the source water is irrelavent.

Theoretically, any buffer that is both fish safe and has a stable pH in the range between about 6.5 and 8.5 could be used to maintain the KH of the pond, but for practical purposes, most people turn to sodium bicarbonate (NaHCO3 – aka “baking soda”). When sodium bicarbonate is used to raise the KH, the resulting pH will be 8.3.

Another alternative is dipotassium phosphate (K2HPO4). The advantage of dipotassium phosphate is that the stable pH is 7.2, which is closer to neutral, and some koi kichi view that as being “ideal”. The two disadvantages of using dipotassium phosphate are that it is far more expensive than sodium bicarbonate [about $4.00 per pound for dipotassium phosphate versus $4.50 per 13.5 pounds (about $0.33 per pound) for sodium bicarbonate], and phosphate will promote algae growth in the pond. Since phosphate has three ionic states (H2PO4- , HPO4--, and PO4---), mixes of mono- and di-potassium (or mono- and di-sodium) phosphate can be used to create a buffer with any pH between 4.8 and 8.8.

2.3. What is the ideal KH?

What the KH should be is a difficult question to answer. The lighter the stocking densities and the less likely it is to have natural events (e.g., heavy rainfall, dust storms, ash from wildfires, etc) affect water quality, the lower the safe limit on KH can be. On the other hand, if the pond is heavily stocked or if you live in an area that is subject to heavy rainfall, then the KH should be higher. Also, bead filters, for some reason, perform better when the KH is higher. The general rule is that KH should be above 100 ppm if you do not have a bead filter and above 150 ppm if you do have a bead filter. Again, if the pond is lightly stocked and it is not likely to be adversely affected by nature, then a KH as low as 80 ppm is probably sufficient. On the other hand, if you are expecting a tropical storm, then it makes sense to raise the KH to 300 ppm to prevent a pH crash when the pond is hit by a foot of rain.

The Japanese koi breeders have relatively low KH and a pH near 7.0; therefore, some koi kichi believe that since the Japanese breeders produce the best koi, it follows that high KH and high pH will be detrimental to the health or appearance of the koi. I have no data to suggest whether there is a true correlation between KH and the quality of the koi. On the other hand, having a stable pH, regardless of what that pH is, will be better for the health of the koi than having wide pH swings would be in an effort to chase the “perfect” pH and KH.

2.4. How does sodium bicarbonate raise the KH and maintain the pH?

As stated above, a mix of a weak acid and its salt forms a buffer. Sodium bicarbonate (NaHCO3) is the salt of carbonic acid (H2CO3), and carbonic acid is formed when carbon dioxide (CO2) is dissolved in water. The pH of a bicarbonate buffer is 8.3.

When sodium bicarbonate is added to water, it disassociates to sodium ions (Na+) and bicarbonate ions (HCO3-). In solution, carbon dioxide, carbonic acid, and bicarbonate are always in equilibrium with each other. If you change the concentration of one of them the other two will change to maintain the equilibrium.

2.4.1. How bicarbonate reacts when H+ is added or OH- is removed from the water

When bicarbonate is present in solution and a H+ is added to the water or an OH- is removed, bicarbonate will pick up the extra H+ to form carbonic acid. Since carbonic acid is in equilibrium with carbon dioxide, a molecule of carbonic acid will dissociate to form CO2 + H2O, and assuming the pond has proper aeration, the extra CO2 is lost to the atmosphere. Since bicarbonate picked up the extra H+, the concentration of H+, and therefore the pH, did not change, but one molecule of bicarbonate was lost. As more H+ is added (or more OH- is removed), more bicarbonate will be used up as it scavenges the free H+. This causes KH to go down, but as long as there is still sufficient bicarbonate present in the solution, the pH will remain at 8.3. Eventually, though, the concentration of bicarbonate will be too low to maintain the pH at 8.3. As that happens, the pH will start to drift downward. Initially, the drift will be slow, but the lower the bicarbonate concentration becomes, the faster the pH will drop.

Some ponders try to add small amounts of sodium bicarbonate to maintain a pH that is lower than 8.3, but no lower than 7.0. While that can be done with careful and frequent (as in daily, or in heavily stocked ponds, two or three times per day) titration of the sodium bicarbonate, there is no room for error. Adding too much sodium bicarbonate will raise the pH back up to 8.3, and not adding bicarbonate frequently enough will result in a rapid fall in pH. For most ponders, it is not a good idea to try to maintain a pH less than 8.3 by using sodium bicarbonate. The risk of a rapid drop in pH is far greater than any benefit there might be in maintaining the pH at some value other than 8.3. If you want a lower pH, then you are forced to use a different buffer, but as I mentioned above, other buffer systems come with their own problems. This is why most ponders who are not blessed with source water with a naturally high KH rely on sodium bicarbonate to maintain the KH and accept that the pH will be 8.3.

2.4.2. How bicarbonate reacts when H+ is removed or OH- is added.

When bicarbonate is present in solution and H+ is removed or OH- is added, bicarbonate will give up a H+ to form carbonate (CO3--). As long as the bicarbonate concentration is higher than the carbonate concentration, the pH will remain at 8.3; however, as carbonate begins to accumulate, carbonate starts to compete for any available H+. As long as the exchange of the H+ is just between bicarbonate and carbonate, there is no net change in the free H+ and OH- concentrations, so the pH will remain at 8.3. As the carbonate concentration climbs; however, carbonate will start to pick up an H+ from water, which will leave an extra OH-. When that happens, the pH starts to go up. Initially, the upward drift will be slow, but as the concentration of carbonate rises with respect to the concentration of bicarbonate, the rise in pH will become faster and faster.

If there are free calcium ions (Ca++) in the water, whenever carbonate is formed, the carbonate will combine with the calcium ion to form calcium carbonate (CaCO3). Calcium carbonate is nearly insoluble at a pH above about 7.2, so when calcium carbonate forms, it precipitates out of the water. Thus, until all of the bicarbonate is depleted, the concentration of bicarbonate will remain much higher than the concentration of carbonate, and the pH will remain at 8.3. Again, as bicarbonate is used up, the KH will go down, but as long as there is much more bicarbonate than carbonate, the pH will not change.

2.4.3. How much sodium bicarbonate do I need to add?

One pound of sodium bicarbonate per 1000 gallons will raise the KH by 85.56 ppm. Thus, if the KH of the source water is 36 ppm and you want the KH of the pond to be 150 ppm, you need to raise the KH by 114 ppm, so you would need to add 1.33 pounds (21.33 ounces, or 603 grams) of sodium bicarbonate per 1000 gallons. If the pH is already between 8.2 and 8.4, then the addition of the sodium bicarbonate will not have any effect on the pH; however, if the pH is lower than 8.2, sodium bicarbonate will bring the pH up to about 8.3. If the pH is low, then only add approximately 10% of the total amount of sodium bicarbonate you need to add every 12 hours until the KH is where you want it to be.

Keep in mind that the biological forces in the pond are constantly using up the KH. The speed at which KH is being utilized will be unique for each pond. Even though in the example above, I calculated that 21.33 ounces of sodium bicarbonate per 1000 gallons would be needed to raise the KH to 150 ppm, if I add 2 ounces per 1000 gallons every 12 hours, there will come a point at which the rate that I am adding KH is equal to the rate that the KH is being utilized, so the measured KH is not increasing. When that happens, increase the amount that is being added at one time by a factor of 1.5. In the example above, that would mean I should start adding 3 ounces of sodium bicarbonate per 1000 gallons until the KH gets to where I want it to be.

Once the KH has reached the range you want (I keep my pond between 160 and 214 ppm or 9 and 12 °dH), keep monitoring the KH to see how quickly it falls. In my case, when the KH drops to 9 °dH, I add enough sodium bicarbonate to bring it back up to 12 °dH. That amount can be added at one time, since the pH is still at 8.3. You will learn quickly how frequently you will need to add sodium bicarbonate to your pond to maintain the KH in the range you want.

In addition to adding sodium bicarbonate to compensate for the KH that was utilized in the pond, when you do a water change, you will have to add enough sodium bicarbonate to compensate for the amount of new water you are adding based on the difference between the KH of the source water and the target KH for your pond. Thus, if you are changing 200 gallons, the KH of the source water is 2 °dH, and you want the pond to be at 12 °dH, you will need to add enough sodium bicarbonate to raise the KH of 200 gallons of water by 10 °dH, or 6.7 ounces. The Koi and Water Garden Society of Central New York have an excellent calculator (http://www.cnykoi.com/calculators/calckh.asp) on their web site.

Some people recommend dissolving the baking soda in a bucket of water and sprinkling it around various parts of the pond. While that makes them feel better, if you have adequate water circulation in your pond, it is not really necessary. The water return in my pond goes through about a 15 foot stream and then down a three-step waterfall before it enters the pond. I just dump the dry sodium bicarbonate at the top of the stream bed. The hourly flow through the stream is about twice the volume of my pond, so the baking soda is distributed throughout the pond quickly. Others dump the powder into the skimmer basket, and that is also a good option. The only things you really need to avoid is dumping the powder directly on the fish or dumping it directly into your biofilter. As long as it is dumped into a high flow area of the pond, it will not cause a problem.

2.4.4. Conclusions regarding bicarbonate

As long as the pond has sufficient aeration, bicarbonate will not let the pH drop below 8.3 when H+ is add or OH- is removed. Bicarbonate by itself can only prevent the pH from rising for a short time when OH- is added or H+ is removed. Only if calcium ions are present can bicarbonate prevent the pH from going up long term. KH should never be allowed do drop below 80 ppm, but for the average ponder, the 150 to 215 ppm range is usually better. Going as high as 300 ppm is not dangerous, and is probably a good thing to do if you are in an area that is about to get hit with a huge amount of rain.

2.5. Can I use oyster shells, crushed coral, or limestone to control the KH and pH?

Oyster shells, crushed coral, and limestone are primarily composed of calcium carbonate. Calcium carbonate in solution will raise both the GH (see the discussion below) and the KH and will raise the pH if the pH is below about 8.0. In areas where the source water comes from limestone wells, the source water will have a reasonably high KH and GH and a stable pH. In this situation, frequent water changes are all that are needed to maintain KH and pH. If the source water is low in KH and has a low pH, it might be possible to run the water over (or through) oyster shells, crushed coral, or limestone to raise the KH and pH. This might be sufficient if you have a low stocking density or if you are doing daily or continuous water changes. The problem comes with the speed at which calcium carbonate will dissolve. The higher the pH, the lower the calcium carbonate solubility is, and it is very likely that in even a moderately stocked pond, relying on calcium carbonate alone will not be sufficient, since calcium carbonate cannot react quickly enough to ward off a pH crash.

When I kept mbuna (African cichlids from Lake Malawi), I maintained the GH, KH, and pH of the aquarium by using crushed coral as the substrate for an under-gravel filter. I had 20 pounds of crushed coral in a 55 gallon aquarium. Taking into account the rocks that were used in the decor and the volume taken up by the coral, there was only 35 gallons of water in the aquarium. The flow rate through the coral was about 80 gallons per hour. That setup worked fine (although vacuuming the gravel two to three times per week was a chore) to keep the KH around 80 and the pH at 8.2, but if you scaled that up to a 3200 gallon koi pond, and take into consideration the forces of nature that affect the water quality of a pond from which the indoor aquarium was sheltered, it would take at least a ton of coral to provide the same degree of stability that I was able to maintain in the aquarium. While a KH of 80 is fine for an indoor aquarium, it is on the low side for a pond. If you use oyster shells, crushed coral, or limestone to maintain KH and pH in your pond, you should still check the KH regularly and be prepared to supplement with sodium bicarbonate.

06-14-2013, 10:14 AM

GH (general hardness or permanent hardness) is a measure of the sum of all divalent (or, more correctly, bivalent) cations dissolved in the water. A divalent cation is any ion or molecule that has two more protons than it has electrons – thus a charge of +2), The most common divalent ions are calcium (Ca+2), magnesium (Mg+2), and iron (II) (Fe+2), but there are many others that can be present in very low concentrations, such as barium (Ba+2), chromium(II) (Cr+2), copper(II) (Cu+2), lead(II) (Pb+2), manganese(II) (Mn+2), mercury(I) (Hg2+2), strontium (Sr+2), tin(II) (Sn+2), and zinc (Zn+2). GH will not tell you which divalent cations are present – GH only tells you the concentration of all of the divalent cations combined.

Like KH, GH can be expressed in either German degrees hardness (°dH) or parts per million (ppm). Just as with KH, 1 °dH is equal to 17.85 ppm. According to the US Geological survey, water with a GH of up to 60 ppm is classified as “soft water”, water with a GH of 61 to 120 ppm is “moderately hard”, water with a GH of 121 to 180 ppm is “hard”, and water with a GH greater than 180 is “very hard”.

3.1. How does GH affect koi?

It is said that koi raised in soft water have stronger beni, and koi raised in hard water have stronger sumi, but also are more likely to develop shimi.

3.2. Should I adjust the GH of my water?

3.2.1. Hard source water

If your source water is naturally high in GH, it is possible (although potentially expensive) to use a water softener unit to lower the GH. These units work by exchanging two sodium ions (Na+) for each divalent cation. Another way to lower the GH is to use reverse osmosis water. The down side of reverse osmosis is that for every gallon of reverse osmosis water that you produce, you waste approximately four gallons of water. If you live in an area where water is expensive or in short supply, reverse osmosis is probably not a good choice.
If you want to show your koi and you have problems with shimi, with your asagi being too dark, or with the beni being too weak, it might be worth it to you to try to soften the water. For most koi keepers, though, trying to lower the GH is not worth the effort. The koi will live long and healthy lives in hard water. Their colors might be different than they would be in softer water, but the hard water does not have an adverse effect on their health.

3.2.2. Soft source water

If you live in an area with a naturally low GH, there might be some advantage to adding calcium chloride and magnesium sulphate (Epsom salts). Some have shown that the addition of one pound of calcium chloride and one pound of Epsom salts per 1000 gallons will shorten the time that it takes for the filter to cycle.

As stated above, if you are using baking soda to maintain the KH of your pond, and if the pH is going above about 8.5, then it makes sense to add one pound of calcium chloride per 1000 gallons so that the Ca+2 can help the bicarbonate maintain the pH in the 8.2 to 8.4 range. If the GH is low, it is certain that the Ca+2 concentration is low; however, even if the GH is high, it is possible that all of the GH is due to Mg+2, Fe+2 or other divalent cations, and there is no Ca+2 present.

I do not recommend that everyone who has a low GH add calcium chloride or Epsom salts to the water, but if you are trying to cycle a new filter, then you might want to add calcium chloride and Epsom salts initially, and let the GH drop over time as you do water changes after the filter has cycled. Similarly, if the pH in your pond has large swings between dawn and late afternoon and the KH is already at 150 ppm or higher, then it is reasonable to add calcium chloride. Once the pond has balanced out and high pH is no longer a problem, then the calcium concentration can be allowed to fall through regular water changes.

If you do add calcium chloride to the pond, dissolve it in water first, and do not add it at the same time as you add sodium bicarbonate. When calcium chloride is added to water, it gives off a lot of heat. If calcium chloride and sodium bicarbonate are added to the water at the same time, they will combine immediately to form calcium carbonate, which will precipitate out of the water. Wait long enough (I recommend at least the time it takes for the pumps to turn over the complete volume of the pond three times) for one of them to be completely distributed before adding the other one.

06-14-2013, 10:21 AM

pH is the measure of the activity of hydrogen ions in solution

KH is the measure of the ability of a solution to resist a change in pH

GH is the measure of all divalent cations that are dissolved in water

Sodium bicarbonate will raise the KH and prevent the pH from falling below 8.3, but by itself, sodium bicarbonate cannot prevent the pH from rising above 8.3.

One pound of sodium bicarbonate per 1000 gallons of water will raise the KH by 85.56 ppm

Sodium bicarbonate will raise the pH to 8.3 if the starting pH is lower than that.

Sodium bicarbonate might or might not drop the pH to 8.3 if the starting pH is higher than that.

Sodium bicarbonate has no effect on GH.

Calcium chloride and magnesium sulphate (Epsom salts) will raise GH but have no effect on KH.

Even though calcium chloride has no effect on KH, the calcium ions added by calcium chloride can work with sodium bicarbonate to prevent the pH from going above 8.3

Calcium chloride will only affect pH if the pH is above 8.3 and sodium bicarbonate has been used to raise the KH.

Magnesium sulphate has no effect on pH.

If you need to change the pH of the pond water, always do so by changing the KH – not by adding acids or bases in an attempt to change the pH directly.

If your source water has high dissolved carbon dioxide concentrations, add an aerator to the end of your hose.

Do not add calcium chloride and sodium bicarbonate to the pond at the same time – separate the timing of the addition by several hours. If you do add calcium chloride and sodium bicarbonate at the same time, they will combine to form calcium carbonate, which will precipitate out of the water.