Aquarium KH: How to Safely Raise and Lower KH Levels

KH (dKH)	Water Stability	Recommendation
0–1	Extremely unstable	Increase KH immediately
2–3	Soft, lightly buffered	Suitable for some soft-water species
4–8	Stable	Ideal for most community aquariums
9–12	Hard, well buffered	Good for livebearers and African cichlids
12+	Very hard	Suitable for some hard-water species

Aquarium KH (carbonate hardness) is one of the most important water parameters in an aquarium because it stabilizes pH. KH is closely related to alkalinity, which determines the water’s ability to buffer acids that are constantly created by various biological and chemical reactions taking place within an aquarium.

What Is KH (Carbonate Hardness)?

Quick Answer: KH (carbonate hardness) measures your aquarium’s buffering capacity, helping stabilize pH. Low KH can lead to dangerous pH crashes, while high KH makes pH more resistant to change. Most freshwater community aquariums do well around 4–8 dKH, although ideal values depend on the species being kept.

KH (carbonate hardness) is used to measure the concentration of carbonate and bicarbonate ions 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 approximately 17.848 mg/L or 17.848 ppm.

Despite the fact that KH and alkalinity are two separate measurements, the terms are frequently used interchangeably in the aquarium hobby. Carbonate alkalinity and KH are both measured in ppm, but they are not always equal. However, in water with a pH below 8.5, KH and carbonate alkalinity are often nearly equivalent, with a difference of approximately 1%.

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

In the aquarium hobby, alkalinity—the ability of water to resist changes in pH caused by acids—is generally 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 carbonate alkalinity and KH are generally close to being equal under typical aquarium conditions.

As KH and carbonate alkalinity increase, the water becomes more resistant to pH fluctuations. 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.

Ideal Aquarium KH Levels

The ideal KH level will vary depending on the type of fish, invertebrate, coral, and other aquatic life being kept. In a reef aquarium, KH is an especially important water parameter because corals consume carbonate as they build their calcium carbonate skeletons.

For many freshwater community aquariums, KH levels between 4 and 8 dKH provide sufficient buffering, whereas reef aquariums are commonly maintained around 7–8 dKH. However, some species require considerably lower or higher KH levels.

Recommended KH ranges for several common freshwater and saltwater aquarium inhabitants:

Species	KH	ppm
Corals	7–8 dKH	125–143 ppm
Cherry shrimp (Neocaridina)	2–4 dKH	35–71 ppm
Ghost shrimp (Palaemonetes)	4–8 dKH	71–143 ppm
Vampire shrimp (Atya)	2–10 dKH	35–178 ppm
South American Cichlids	2–4 dKH	35–71 ppm
Lake Malawi Cichlids	10–12 dKH	178–214 ppm
Lake Tanganyika Cichlids	10–12 dKH	178–214 ppm
Lake Victoria Cichlids	10–12 dKH	178–214 ppm
Discus	1–3 dKH	18–54 ppm
Amazonian Fish	1–4 dKH	18–71 ppm
Livebearers	4–8 dKH	71–143 ppm
Corydoras	3–10 dKH	54–178 ppm
Betta Fish	3–8 dKH	54–143 ppm

KH values between 4 and 8 dKH will often provide sufficient buffering for many freshwater community aquariums. However, the ideal KH range can vary depending on the species being kept and the aquarium conditions.

Research highlight: A 2014 study found that total alkalinity declined as corals and other reef organisms deposited calcium carbonate during calcification. In coral-only experiments, the amount of alkalinity used closely matched the calcium being deposited, demonstrating why growing corals can steadily deplete alkalinity in reef aquariums.

How to Raise KH in an Aquarium

Quick Answer: KH can be raised gradually with crushed coral, aragonite or limestone, while a commercial KH buffer provides a more controlled adjustment. Baking soda can provide a quick, temporary increase when only a minor adjustment is needed. To minimize stress, make changes gradually and test KH and pH throughout the process.

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 amount 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 raise KH and strengthen the water’s buffering capacity.

Raise KH With Water Changes

Water sources can contain varying levels of KH. If your tap or replacement water has a higher KH than your aquarium, regular water changes can help replenish depleted alkalinity and restore buffering capacity. Alternatively, you can replace 20–30% of the aquarium water with mineralized water that has a higher KH than the aquarium.

Aquariums with a lot of fish may require more frequent water changes with a greater percentage of water replaced. Heavily stocked aquariums generally require larger or more frequent water changes than lightly stocked aquariums, although the appropriate schedule depends on stocking levels, filtration, feeding, and water quality. This is particularly true in aquariums lacking sources of rocks that can release sufficient 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 very low, maintain a rigorous weekly water change schedule and supplement with the additional methods listed below to raise your KH levels.

Raise KH With 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.

Aragonite can also gradually increase KH as it dissolves, although the effect will depend on the aquarium’s water chemistry. When attempting to prevent KH from becoming too low and maintain a more stable pH, using aragonite may be helpful.

Raise KH With an Aquarium KH Buffer

KH buffers are offered in most pet stores and online. Commercial KH buffers can effectively increase alkalinity, although regular dosing may be necessary to maintain the desired KH level. KH buffers can be useful when breeding soft-water fish or when a specific KH and pH range needs to be maintained.

Buffers are useful for controlled KH adjustments and for maintaining stable KH levels in aquariums where alkalinity is regularly depleted. Some aquariums, particularly reef systems, may require routine buffering, while unexpected or persistent KH loss in other aquariums may require investigating the underlying cause.

Raise KH With Dolomite & Limestone

Dolostone (dolomite) and limestone contain calcium, magnesium, and carbonate compounds. As these rocks slowly dissolve, they release minerals and carbonates into the water, which can increase KH and GH.

One method of gradually increasing KH is to place crushed dolostone or limestone in a media bag inside the aquarium filter while regularly 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.

Because dolostone and limestone dissolve more slowly when they are not placed directly in the flow of your filtration system, adding larger pieces directly to the aquarium can provide a gradual way to help maintain KH levels.

Raise KH With Baking Soda (Sodium Bicarbonate)

Pure sodium bicarbonate (baking soda) can increase KH without directly increasing GH. Approximately one teaspoon of baking soda per 13.5 US gallons of water can increase KH by about 4 dKH. Sodium bicarbonate solutions are mildly alkaline and typically have a pH of approximately 7.9–8.4, depending on concentration and testing conditions.

Potassium bicarbonate can also be used to increase KH. It provides bicarbonate alkalinity similarly to sodium bicarbonate but adds potassium rather than sodium, which may make it a preferred alternative in some planted freshwater aquariums. Because potassium bicarbonate and sodium bicarbonate have different molecular weights, they should not be dosed at identical amounts by weight.

Baking soda is best used for controlled KH adjustments rather than as a long-term substitute for a properly buffered water source. Sodium bicarbonate is generally safe for aquarium fish at low concentrations. When it dissolves in water, it separates into sodium and bicarbonate ions. The bicarbonate increases KH, while the sodium remains in the water and can gradually accumulate with repeated dosing. High sodium concentrations or large additions of sodium bicarbonate can become harmful, so KH should be raised gradually while monitoring KH and pH.

Add baking soda while performing a small 10% water change. Calculate the required amount based on the aquarium’s total water volume and the desired increase in KH, then mix the measured baking soda into a separate bucket of dechlorinated water before slowly adding it back to the aquarium.

If baking soda is needed regularly to maintain KH, the underlying cause of the KH loss or the buffering capacity of the source water should be addressed. Regular water changes will dilute sodium that accumulates with repeated sodium bicarbonate use.

Amount of baking soda required to raise KH based on total aquarium water volume:

KH Increase (ppm)	KH Increase (dKH)	5 Gallons	10 Gallons	20 Gallons	40 Gallons
10 ppm	0.56 dKH	340 mg	680 mg	1.36 g	2.72 g
20 ppm	1.12 dKH	680 mg	1.36 g	2.72 g	5.44 g
30 ppm	1.68 dKH	1.02 g	2.04 g	4.08 g	8.16 g
40 ppm	2.24 dKH	1.36 g	2.72 g	5.44 g	10.88 g
50 ppm	2.80 dKH	1.70 g	3.40 g	6.80 g	13.60 g
60 ppm	3.36 dKH	2.04 g	4.08 g	8.16 g	16.32 g
70 ppm	3.92 dKH	2.38 g	4.76 g	9.52 g	19.04 g
80 ppm	4.48 dKH	2.72 g	5.44 g	10.88 g	21.76 g
90 ppm	5.04 dKH	3.06 g	6.12 g	12.24 g	24.48 g
100 ppm	5.60 dKH	3.40 g	6.80 g	13.60 g	27.20 g

Note: The 5, 10, 20 and 40-gallon columns refer to the aquarium’s total water volume, not the volume of replacement water. When performing a 10% water change, dissolve the listed amount of baking soda in a separate container of replacement water before slowly adding it to the aquarium. Do not add baking soda directly to the aquarium without premixing.

Causes of Low Aquarium KH

Quick Answer: Carbonate hardness (KH) gradually declines as biological and chemical processes consume alkalinity in an aquarium. Common causes include nitrification, fish waste, decomposing organic matter, low-KH source water, and, in reef aquariums, alkalinity consumption by corals and other calcifying organisms.

Acids are continuously produced by several processes within an aquarium, including the nitrogen cycle, fish respiration and waste, and the decomposition of organic matter. Over time, these processes deplete alkalinity, reducing the water’s buffering capacity and potentially causing pH to decline.

The nitrogen cycle is an important contributor to this process. During nitrification, beneficial bacteria convert ammonia into nitrite and then nitrate, consuming alkalinity along the way. In reef aquariums, corals and other calcifying organisms also consume alkalinity as they build their calcium carbonate skeletons and structures.

Regular water changes can help replenish alkalinity; however, aquariums with naturally low-KH source water or reef systems with higher alkalinity demands may require additional methods to maintain stable KH and pH levels.

Research highlight: Research on recirculating aquaculture systems has demonstrated the close relationship between alkalinity and biological filtration. Nitrification consumes alkalinity as ammonia is oxidized to nitrite and nitrate, and insufficient alkalinity can contribute to declining pH and reduced nitrification efficiency. In a 2024 study, higher alkalinity improved ammonia and nitrite removal by biological filters compared with lower-alkalinity conditions.

How to Lower KH in an Aquarium

Quick Answer: The safest way to lower KH in an aquarium is to dilute high-KH tap water with reverse osmosis (RO), distilled, or demineralized water during water changes. Peat moss, driftwood, and other sources of tannins can also gradually lower KH. Reduce KH slowly and monitor pH because lowering KH also reduces the aquarium’s buffering capacity.

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 water is strongly buffered and resistant to pH changes. When source water has a very high KH, diluting it with water that contains fewer dissolved minerals is often the most practical way to lower KH.

Other methods can lower KH, although water changes may increase it again if the source water has a higher KH.

Lower KH With Peat Moss or Sphagnum Moss

Other than diluting source water, peat moss and sphagnum moss are natural methods that can help lower and maintain aquarium KH levels.

Peat moss and sphagnum moss release organic acids, including tannic acids, which may gradually lower pH and reduce KH under suitable water conditions. Peat moss may also discolor water; to lessen the impact of staining, soak it in a bucket of water for 24 to 48 hours.

Peat moss or sphagnum moss can be placed in a media bag inside the aquarium filter, where water continuously passes through it. As the moss releases organic acids and tannins, KH and pH can gradually decline. Peat may also discolor the water, so start with a small amount and test the water regularly to make sure the change does not occur faster than intended.

Another option is to pre-soak the moss in a separate container of water for 24–48 hours. Test the KH and pH before using the treated water during an aquarium water change. This method provides more control because you can test the water before it is added to the aquarium.

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.

Lower KH With Distilled or 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. Because distilled and demineralized water contain very few dissolved minerals, they are typically mixed with mineralized tap water or remineralized before use. The appropriate ratio depends on the KH, GH, and other water parameters required by the species being kept.

The table below shows the estimated KH after dilution with distilled or demineralized water:

Starting KH	20% Dilution	40% Dilution	50% Dilution	60% Dilution	80% Dilution
6 dKH	4.8	3.6	3.0	2.4	1.2
8 dKH	6.4	4.8	4.0	3.2	1.6
10 dKH	8.0	6.0	5.0	4.0	2.0
12 dKH	9.6	7.2	6.0	4.8	2.4
14 dKH	11.2	8.4	7.0	5.6	2.8
16 dKH	12.8	9.6	8.0	6.4	3.2
18 dKH	14.4	10.8	9.0	7.2	3.6

Lower KH Naturally With Tannins

Tannic acid is mildly acidic, with dilute solutions commonly measuring around pH 6.0. As tannins are released into aquarium water, they can gradually consume some of the water’s buffering capacity and lower pH. This effect is most noticeable in soft water with a low KH. In hard water with a high KH, the greater buffering capacity resists changes in pH, so tannins generally produce a much smaller effect.

Tannins are gradually released from organic materials such as driftwood, bogwood, and aquarium-safe leaves, including Indian almond (catappa) and beech leaves.

Lower KH With Reverse Osmosis (RO) Water

A semipermeable membrane is used in the reverse osmosis method of water filtration to extract ions, undesirable compounds, and larger particles from water. Most, if not all, 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.

RO water typically contains no measurable KH or GH, although trace amounts of minerals may remain. For this reason, it is commonly mixed with mineralized tap water or remineralized before being added to an aquarium.

The table below shows the estimated KH after mixing source water with different percentages of RO water:

Starting KH	20% (RO)	40% (RO)	50% (RO)	60% (RO)	80% (RO)
6 dKH	4.8	3.6	3.0	2.4	1.2
8 dKH	6.4	4.8	4.0	3.2	1.6
10 dKH	8.0	6.0	5.0	4.0	2.0
12 dKH	9.6	7.2	6.0	4.8	2.4
14 dKH	11.2	8.4	7.0	5.6	2.8
16 dKH	12.8	9.6	8.0	6.4	3.2
18 dKH	14.4	10.8	9.0	7.2	3.6

Note: These estimates assume the RO water has a KH of 0 dKH. If measurable KH remains in the RO water, the resulting KH will be slightly higher.

Testing KH Levels

Quick Answer: Aquarium KH can be tested using a liquid test kit or a digital alkalinity checker. For freshwater aquariums, the API GH & KH Test Kit is a popular option that measures KH in dKH using a simple drop-count method. For saltwater and reef aquariums, the Salifert KH/Alkalinity Test Kit is commonly used to measure alkalinity through titration. Digital alkalinity checkers are also available for both freshwater and marine aquariums.

There are several ways to test carbonate hardness (KH) in an aquarium. The appropriate test kit or digital checker will depend on whether you have a freshwater or saltwater aquarium.

Testing KH Levels in Freshwater Aquariums

To test KH in a freshwater aquarium, many aquarists use a liquid dropper test kit, such as the API GH & KH Test Kit. This kit includes two test solutions: one for measuring KH and another for measuring GH. Alternatively, a freshwater digital colorimeter can be used to measure alkalinity in parts per million (ppm), which may be convenient if you test your water regularly.

To test your freshwater aquarium’s KH with the API drop-style test kit, follow these steps:

• Fill a clean test tube with aquarium water to the 5 mL line. Avoid underfilling or overfilling the tube, as this may affect the accuracy of the results.
• Hold the KH test solution bottle vertically and add one drop to the test tube.
• Replace the cap and gently shake the test tube. The water should initially turn blue.
• Continue adding the KH solution one drop at a time, gently shaking the test tube after each drop. Keep count of the total number of drops added.
• Once the water changes from blue to bright yellow, stop adding the solution. The total number of drops used represents the KH level in degrees of carbonate hardness (dKH).
• To convert dKH to parts per million (ppm), multiply the dKH value by 17.9.

Testing KH Levels in Saltwater Aquariums

To test KH in a saltwater aquarium, many aquarists use a liquid titration test kit, such as the Salifert KH/Alkalinity Test Kit, or a marine digital colorimeter that measures alkalinity directly in dKH.

The Salifert KH/Alkalinity Test Kit includes a KH indicator reagent, KH titration reagent, 5 mL syringe for measuring aquarium water, 1 mL titration syringe, test vial, plastic syringe tip, and instructions with a KH/alkalinity conversion chart.

To test the KH level in your saltwater or reef aquarium using the Salifert KH/Alkalinity Test Kit, follow these steps:

• Using the 5 mL syringe, measure 4 mL of aquarium water and transfer it to the test vial.
• Gently shake the KH indicator reagent bottle, then add 4 drops to the test vial.
• Gently swirl the vial for approximately 5 seconds. The water should turn blue or blue-green.
• Attach the plastic tip to the 1 mL syringe and draw 1 mL of KH titration reagent. Position the lower end of the black syringe plunger at the 1.00 mL mark.
• Add the titration reagent to the test vial one drop at a time, gently swirling the vial after each drop.
• Continue adding the reagent until the water changes from blue or blue-green to orange-red or pink, whichever color appears first.
• With the syringe tip pointing upward, read the position of the upper end of the black portion of the plunger. The syringe is graduated in 0.01 mL increments.
• Use the conversion chart provided with the test kit to determine the aquarium’s KH in dKH or alkalinity in meq/L from the amount of titration reagent remaining in the syringe.

The table below provides Salifert’s mL-to-dKH conversion chart:

mL	dKH	mL	dKH	mL	dKH	mL	dKH	mL	dKH
0.00	15.7	0.20	12.5	0.40	9.3	0.60	6.1	0.80	2.8
0.02	15.3	0.22	12.1	0.42	8.9	0.62	5.7	0.82	2.5
0.04	15.0	0.24	11.8	0.44	8.6	0.64	5.4	0.84	2.2
0.06	14.7	0.26	11.5	0.46	8.3	0.66	5.1	0.86	1.9
0.08	14.4	0.28	11.2	0.48	8.0	0.68	4.8	0.88	1.6
0.10	14.1	0.30	10.9	0.50	7.7	0.70	4.5	0.90	1.2
0.12	13.7	0.32	10.5	0.52	7.3	0.72	4.1	0.92	0.9
0.14	13.4	0.34	10.2	0.54	7.0	0.74	3.8	0.94	0.6
0.16	13.1	0.36	9.9	0.56	6.7	0.76	3.5	0.96	0.3
0.18	12.8	0.38	9.6	0.58	6.4	0.78	3.2	0.98	0

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