KH, aka carbonate hardness, is a measure of the ability of water to resist a drop in pH. Maintaining a stable pH is important to the overall health and well being of the fish that live in our ponds. It does not matter whether we keep koi, goldfish, or any other variety of fish, nearly every species of fish fares better when the pH is stable, and it is KH that keeps the pH stable.

KH can be expressed in parts per million (ppm or mg/L) or in German degrees of carbonate hardness (dKH). One dKH is equal to 17.848 ppm.

There are inexpensive test kits available from API and other companies that measure KH based on the number of drops of reagent that must be added to a vial of water to make the color change from blue to yellow. Each drop of reagent that is added is equal to one dKH, so if it takes 10 drops to change the water to yellow, the KH is 10 dKH or approximately 178 ppm.

The biological activity in the pond and other forces of nature (acid rain, leaves and other debris falling into the pond, etc) are constantly producing acid, which forces the pH to go down. Biological activity includes the biofilter.

An adequate KH will prevent the pH from going down, but the KH will be consumed in the process; thus, it is important to check the KH on a regular basis so that the KH can be maintained at a level that will prevent the pH from dropping. Everyone should also test the KH of their source water. If the source water has a high KH, then regular partial water changes might be all that is needed to maintain the KH of the pond; however, if the KH of the source water is low, other means must be taken to raise the KH of the pond, and a partial water change might cause the KH to drop.

One of the most effective and inexpensive means to raise the KH of the pond is by adding baking soda (sodium bicarbonate). If the pH of the pond is less than 8.3, the addition of baking soda will raise the pH to 8.3, but will have little effect on the KH. Once the pH reaches 8.3, one pound of baking soda will raise the KH of 1000 US gallons of water by 4.8 dH (85.6 ppm). Baking soda can be bought at places like BJís. Costco, or Samís Club for as little as $5.00 for a 13.5 pound bag.

Since it is important that pH is not changed quickly, baking soda must be added in small amounts over several days if the pH is below 8.3 so that the pH rises slowly. If the pH is already at 8.3, then the amount of baking soda needed to bring the KH to the desired value can be added at one time.

Although baking soda is inexpensive and extremely effective at raising KH and preventing the pH from dropping, baking soda by itself will not prevent the pH from going above 8.3. This is usually not a problem, but in ponds that have not yet cycled (or that have had a major disruption to the filter) or in ponds with excessive algae problems, the pH can go up during the day and down at night. In these situations, it is necessary to add calcium chloride in addition to baking soda (but not at the same time Ė separate the additions by several hours). Baking soda will always prevent the pH from going below 8.3, but it can only prevent the pH from going above 8.3 if sufficient calcium is present in the water.

There are many factors that determine how high the KH should be. In general, the higher the stocking density, the less frequent the water changes, and the more rain the pond is subjected to, the higher the KH needs to be. Many will point out that the best quality koi come from Japan and have been grown in mud ponds that have extremely low KH. While this is true, these ponds either have constant flow through of fresh water or they fewer than 1 koi per 1000 gallons of water (and often times they have much more water than that per koi). In a pond with 1 koi per 250 gallons of water and weekly water changes that average 20% or less, the KH should be at least 90 ppm (5 dKH or 5 drops on the test kit). Because enclosed filters such as bead filters and certain types of moving bed filters have low concentrations of oxygen, they tend to produce more acid, which increases the rate at which the KH is consumed. These filters perform better and the pond is more stable when the KH is kept above 160 ppm (9 dKH). If your pond is in an area that is subjected to heavy rainfall, then you should consider bumping the KH to 200 or even 300 ppm (11 to 17 dKH) when a storm is approaching.

How frequently the KH should be measured is also quite variable. In a new pond that has not yet cycled fully, it is a good idea to measure the KH daily. Once the cycling is complete and you will quickly determine how quickly the KH drops in your pond and you can adjust the frequency with which you measure the KH accordingly. You should be sure to measure the KH after every water change, a major rain storm, or any other event that could alter the stability of the pond (e.g., the addition of new fish, treatment with any medication, etc). When the KH gets down to the minimum recommended for your pond, add enough baking soda to increase the KH by 2 dKH, For example, I have a 3200 gallon pond with a bead filter, and I live in an area that is subject to tropical storms. From April through October, I set the bottom at 10 drops. When the KH gets down to 10 drops, I add 1.34 pounds of baking soda, which will bring the KH back up to 12 drops. From November through March, I will let the bottom drop to 9 drops, but I will still add 1.34 pounds of baking soda whenever the KH gets down to 9 drops. When a tropical storm is approaching (it is not unusual to get 10 to 12 inches of rain from one of these), I will measure the KH and then add enough baking soda to take the KH up to about 14 drops, although if I overshoot that, it will not be harmful to the fish.