Aquarium KH: How to Safely Lower or Raise KH Levels

Aquarium KH levels, or rather alkalinity, control the water’s ability to buffer acids that are constantly created by different reactions taking place inside an aquarium. Knowing the GH, KH, and pH levels of your tap water will help you keep healthy fish, plants, invertebrates, and corals.

What is KH

KH (carbonate hardness) is used to determine how many carbonate and bicarbonate anions, such as calcium carbonate and magnesium carbonate, are present in a particular volume of water. KH can be measured in parts per million (ppm), milligrams per liter (mg/L), or degrees of KH (dKH). One dKH is equivalent to 17.848 mg/L or 17.848 ppm.

Despite the fact that KH and alkalinity are two separate measurements, they are frequently used interchangeably. Carbonate alkalinity and KH are both measured in ppm, but they are not always equal. In contrast, KH will be nearly equal to carbonate alkalinity in water with a pH below 8.5 with approximately a 1% inaccuracy.

  • Carbonate Alkalinity CA (mg/L) = [HCO3] + 2 x [CO32−]
  • Carbonate Hardness CH (mg/L) = [HCO3] + [CO32−]

In the aquarium hobby, alkalinity—the power of water to tolerate acidic changes in pH—is of greater interest. Alkalinity, commonly referred to as buffering capacity, is the water’s capacity to neutralize acids and maintain a stable pH. We measure KH because, in most cases, carbonate alkalinity and KH are typically close to being equal.

Since higher concentrations of carbonate alkalinity increase with KH, water is significantly more resistant to pH fluctuations as a result. However, the lower your aquarium’s KH, the lower the carbonate alkalinity, which can lead to a drop in pH once the buffering capacity reaches its limit. As a result, the buffering capacity of alkalinity determines how much acid can be added to a body of water without causing a significant pH shift.

Acids are continuously produced by a number of processes in aquariums, including the nitrogen cycle, decomposing organic matter, fish, etc. Because of this, alkalinity can also be depleted over time; if this occurs, the pH of the water in an aquarium can drop dramatically. Regular water changes replenish alkalinity, but since most tap water is rather soft, there may be times when we must turn to alternative methods.

Ideal KH Level For Freshwater & Saltwater Aquariums

The ideal KH level will vary depending on the type of fish, invertebrate, coral, and so on. In a reef tank, KH is one of the most important water parameters to sustain corals since they absorb carbonates to grow.

For freshwater aquariums, the ideal KH levels range between 4 and 8 dKH, whereas the range for saltwater aquariums is between 7 and 8 dKH. However, some species do require either an exceptionally low or high concentration of KH. Refer to the table below for more information:

Corals7 – 8 dKH125 – 143 ppm
Cherry shrimp (Neocaridina)2 – 4 dKH35 – 71 ppm
Ghost shrimp (Palaemonetes)4 – 8 dKH71 – 143 ppm
Vampire shrimp (Atya)2 – 10 dKH35 – 178 ppm
South American Cichlids2 – 4 dKH35 – 71 ppm
Lake Malawi Cichlids10 – 12 dKH178 – 214 ppm
Lake Tanganyika Cichlids10 – 12 dKH178 – 214 ppm
Lake Victoria Cichlids10 – 12 dKH178 – 214 ppm
Discus1 – 3 dKH18 – 54 ppm
Amazonian Fish1 – 4 dKH18 – 71 ppm
Livebearers4 – 8 dKH71 – 143 ppm
Corydoras3 – 10 dKH54 – 178 ppm
Betta Fish3 – 8 dKH54 – 143 ppm

Minimum KH values of between 4 and 8 will often be sufficient to stabilize a standard home aquarium’s pH levels. The table below provides a reasonable range of GH levels to KH levels, however since these levels can fluctuate greatly, you shouldn’t be alarmed until they are very far off.

Very Soft0 – 4 dGH0 – 4 dKH
Soft4 – 8 dGH4 – 7 dKH
Moderately Hard8 – 12 dGH7 – 8 dKH
Hard12 – 18 dGH9 – 12 dKH
Very Hard18 – 30 dGH12 – 20 dKH

How to Raise KH in Aquariums

If your pH is consistently dropping and becoming more acidic, you may need to test your aquarium’s KH levels. KH serves as a buffer; when your KH levels are insufficient to counteract the number of acids being produced in your tank, this buffer essentially collapses, lowering pH along with it.

Increasing the concentration of dissolved carbonates and bicarbonates will increase your water’s buffering capacity, and a lower KH results in a low buffering capacity. The best methods to raise KH in aquariums are as follows:

Water Change

All sources of water contain varied levels of KH, so the best strategy to restore KH’s buffering capacity is to replace its depleted alkalinity levels. Alkalinity gradually depletes as it continuously neutralizes acids created by numerous processes in your aquarium.

Alternately, you can add 20 to 30 percent of mineralized water to your tank, whether it is store-bought or obtained from a friend’s or relative’s home where the water has a greater mineral concentration.

Aquariums with a lot of fish may require more frequent water changes with a greater percentage of water replaced. Weekly water changes for fully stocked tanks should be 50%, whereas water changes for sparsely filled tanks might only need to be 20 – 25%. This is particularly true in aquariums lacking sources of rocks that can sufficiently produce enough carbonates to raise KH levels.

Since demineralized water will contain very little to no KH, if any type of demineralized water is being used, such as reverse osmosis, the amount that you are using may need to be reduced. If the KH levels in your tap water are really low, maintain a rigorous weekly water change schedule and supplement with the additional methods listed below to raise your KH levels.

Dolomite & Limestone

Magnesium and calcium are found in dolostone (dolomite) and limestone, and when these rocks erode, they produce magnesium, calcium, and carbonates, which are largely responsible for the KH and GH present in all bodies of water.

The best method to increase KH levels is to add one or two bags of crushed dolostone (dolomite) and limestone to your aquarium’s filtration unit while continuously testing the water until you’ve achieved your desired results. Dolostone and limestone have the ability to raise your aquarium’s KH, GH, and pH levels; add or remove some if levels are rising too quickly or too slowly.

As erosion will occur more slowly when they are not directly placed in the current of your filtration unit, adding some large pieces of dolostone or limestone into your aquarium is an excellent approach to maintaining your aquarium’s KH levels.

Crushed Coral, Oyster Shells & Aragonite

Oyster shells and crushed coral contain significant amounts of calcium and magnesium carbonates. Calcium, magnesium, and carbonates are released when they dissolve in the presence of acidic water. While carbonates will raise KH, calcium and magnesium minerals will raise GH.

Although the increase will be relatively minor, aragonite will marginally raise KH levels. When attempting to maintain very low KH levels to regulate pH levels, using aragonite may be helpful.

Aquarium KH Buffers

KH buffers are offered in most pet stores and online. Although this approach is successful at increasing alkalinity, KH levels must be regularly dosed each week to be maintained. It’s recommended to use while breeding soft water species of fish while maintaining a certain pH level.

The use of buffers is best used for breeding fish and emergency purposes. For any other purpose, it should only be used as a temporary solution until you’ve stabilized KH levels more naturally.

Sodium Bicarbonate (NaHCO3)

The addition of sodium bicarbonate without any additives, also known as baking soda, will increase KH without increasing GH. The KH is increased by approximately 4 dKH with one teaspoon for every 13.5 US gallons; this typically results in a minor pH increase as well but does not exceed 8.2 pH.

This method should only be used during emergencies for short periods of time. Sodium bicarbonate is not toxic to fish at low levels; however, when sodium bicarbonate breaks down, sodium ions remain and these levels will continuously rise with the addition of more sodium bicarbonate. High levels of sodium bicarbonate and sodium ions can become toxic.

This method can be used to maintain pH levels for a brief period of time while you work to correct your pH crash or low KH levels. Depending on your fish stock levels, you’ll likely need to add small amounts every other day. To lower the sodium levels when utilizing this strategy, it is advised to perform 50% water changes per week without the addition of sodium bicarbonate.

It is advised to carry out a 10% water exchange and add enough sodium bicarbonate to raise the levels by 10 ppm of the water you are replacing; do not add sodium bicarbonate based on the total amount of water in your tank. Add very small amounts while testing the water to prevent abrupt changes that may shock your aquarium’s inhabitants. The chart below illustrates how much baking soda is required to raise KH concentration by increments of 10 ppm.

Increase by ppm5 Gallons10 Gallons20 Gallons40 Gallons
10 ppm340 mg680 mg1.36 g2.72 g
20 ppm680 mg1.36 g2.72 g5.44 g
30 ppm1.02 g2.04 g4.08 g8.16 g
40 ppm1.36 g2.72 g5.44 g10.88 g
50 ppm1.70 g3.40 g6.80 g13.60 g
60 ppm2.04 g4.08 g8.16 g16.32 g
70 ppm2.38 g4.76 g9.52 g19.04 g
80 ppm2.72 g5.44 g10.88 g21.76 g
90 ppm3.06 g6.12 g12.24 g24.48 g
100 ppm3.40 g6.80 g13.60 g27.20 g

How to Lower KH in Aquariums

If you’re dealing with hard tap water, you might need to lower the KH. For instance, well water is frequently mineral-rich and contains significant amounts of GH and KH.

High KH concentrations in hard water make it very resistant to change, and in some circumstances, the only option is to mix your tap water with other sources of water with lower mineral content.

All other methods are used to gradually lower KH levels while maintaining slightly lower levels, and as soon as you replace water, those levels will rise once more.

Distilled & Demineralized Water

Distilled water is created by heating water to a high temperature and collecting the vapor in a separate container before condensing it back into liquid. Most minerals and contaminants are removed during this process.

Depending on the ratio of distilled or demineralized to tap water used, a mixture of 50% tap water and 50% distilled or demineralized water will reduce the total KH by half. Since an aquarium needs some GH and KH to operate at its best, 70% distilled or demineralized water should be the maximum amount used.

(RO) Reverse Osmosis Water

A partially permeable membrane is used in the reverse osmosis method of water filtration to extract ions, undesirable compounds, and larger particles from water. 99% of the mineral content is successfully removed during this process.

Depending on the ratio of RO water to tap water used, a mixture of 50% tap water and 50% RO water will reduce the total GH and KH by half. Since an aquarium needs some GH and KH to operate at its best, 70% RO water should be the maximum amount used.

Peat Moss & Spaghnum Moss

Other than diluting tap water, peat moss and sphagnum moss are the two best ways to lower and maintain your aquarium’s KH levels.

As minerals are absorbed, peat moss and sphagnum moss releases gallic and tannic acids, which have the effect of lowering pH, GH, and KH. Peat moss can also discolor water; to lessen the impact of staining, soak it in a bucket of water for 24 to 48 hours.

Peat moss and spaghnum moss can be used in a number of ways to lower an aquarium’s KH. One or two bags full of moss can be placed directly into the aquarium filter, added to the tank, or pre-soaked in a bucket of water before using the altered water to refill the aquarium during tank maintenance. Or any combination of the aforementioned.

It is possible to leave peat and sphagnum moss in a bucket of water for months as it continuously develops and decomposes, lowering the KH, GH, and pH of the water in the process. Use the water from the bucket and pour it into your aquarium and then refill the bucket of java moss or sphagnum moss with water for the next use.

Frequently monitor pH, GH, and KH levels of both your aquarium and the bucket of water in which the moss is pre-soaked throughout this procedure. To avoid pH or GH shock in fish, water changes should be performed more often while replacing a smaller volume of water. The results will normally be consistent once a routine is established, so you won’t need to check the water parameters as regularly.

Tannic Acid

Tannic acid has a base pH value of 6.0 and is capable of reducing the KH, GH, and pH in an aquarium to a minor extent. The use of tannic acid is more effective in softer waters with a lower pH, when in hard waters with a high pH, it may only reduce the KH slightly.

Tannic acids will be released as organic materials such as driftwood, bogwood, and aquarium-safe leaves like almond, catappa, or beech decompose. When combined with the other methods outlined above, this method works best for making minor tweaks.


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