How To Raise KH in Freshwater Aquariums [Guide & Tips]

One of the parameters to monitor in fish keeping is KH which is the same as carbonate hardness. KH is a term commonly used in water chemistry. Understanding KH is very important to keep your aquarium fish healthy and flourishing.

Carbonate hardness functions as a blocker to cushion the effects of naturally occurring acids in the tank. But a very low KH level will ultimately lead to fluctuations in the pH level of your aquarium. Sadly, raising the KH level will raise the pH level as well. So it is necessary to know how to raise KH level.

This article discusses what you need to know about raising the carbonate hardness level of your freshwater aquariums while maintaining a stable pH level.

What Is KH (Carbonate Hardness)?

KH stands for carbonate hardness, a water parameter that measures how much dissolved carbonates (CO3) and bicarbonates (HCO3) are present in your aquarium water. It can also be indicated as dKH (degree of carbonate hardness).

Why Does KH Matter in a Freshwater Aquarium?

KH is one of the most critical water parameters that aquarists need to pay attention to. Here are three compelling reasons why KH matters in a freshwater aquarium.

Prevent a pH Crash

KH affects the resistance of your aquarium water to pH variations.

Acidic compounds in the aquarium, such as nitrates and nitrates, are the natural results of the nitrogen cycle in your aquarium’s ecosystem.

Carbonate hardness (KH) neutralizes these naturally occurring acids to prevent any sudden pH variations in the aquarium, as this would be damaging to your aquarium fish’s health.

Related: How to Lower pH Levels in Aquariums

To Buffer Against pH Fluctuation

A low KH level means that your aquarium water has less water buffering capacity and is less resistant to pH variations. Aquarium water with little or no dissolved carbonates and bicarbonates is likely to experience a sudden drop in the pH level.

This could be fatal to your aquarium fish, as a sudden and rapid change in the water’s pH level could lead to serious health issues and even death.

A high KH level increases the water’s buffering capacity. This means that aquarium water has a high amount of dissolved carbonates and bicarbonates, which makes it resistant to pH fluctuations.

The water’s buffering capacity is increased as the pH level is less likely to change suddenly because the high amount of dissolved carbonates and bicarbonates will continue to neutralize the naturally materializing acids in the tank.

To Provide for the Needs of Certain Fish Species

Some freshwater fish species are native to water bodies with high KH levels.

Fish like the African cichlids will not just survive but will flourish in the aquarium water with a high KH, especially in the range of 9-12 dKH.

While many aquarium fish may adapt to the recommended KH range for freshwater aquariums (4-8dKH), it would be impossible to breed fish species that prefer hard water with high KH levels in such waters.

Aquarium plants may also benefit from a higher level of carbonate hardness in the aquarium, especially if the increase is a replication of the KH level in their natural habitats.

What Causes KH To Drop in a Freshwater Aquarium?

how to raise kh in aquarium

Excess Carbon Dioxide (CO2)

Carbon dioxide is crucial for the growth and development of aquarium plants. Aquarium plants also rely on carbon dioxide and light to produce food through photosynthesis.

Aquarists often supplement the fish tanks with carbon dioxide so that plants can thrive and flourish in the aquarium.

However, when there is excess carbon dioxide in your fish tank, KH will drop. The extra carbon dioxide dissolves into carbonic and will make the water in the aquarium acidic if the aquarium plants do not use it up.

Although the KH will initially buffer the high acid level in the water, it will ultimately drop after being eaten away by the excess carbon dioxide.

After the drop in the KH, your tank will also experience a dip in the pH level.

Overcrowded Aquarium

Overcrowded aquariums can also cause a drop in the KH levels. Stocking a fish tank up to 80% of its capacity and beyond is another reason for the drop in KH levels. Overcrowding a tank increases the rate at which the naturally occurring acids in the aquarium are being produced.

You can expect a drop in the KH level of an overcrowded tank because the excess naturally occurring acids will eventually use up a significant portion of the dissolved carbonates and bicarbonates in the water.

This will ultimately lead to a drop in KH levels and a pH crash since KH levels will no longer be sufficient to ensure pH stability.

Irregular Water Changes Routine

Dissolved carbonate and bicarbonate are carbonate hardness resources that can be used up. Carbonate hardness is used up in the conversion of fish waste into ammonia, nitrites, and nitrates by some beneficial bacteria. These bacteria are called nitrifying bacteria.

So the concentration of dissolved carbonate and bicarbonate must be replenished to maintain an optimal KH level. This maintenance is one of the benefits of regular water changes. Frequent water changes must be performed to replenish the depleted carbonate hardness.

How To Raise KH in Freshwater Aquariums Recommended Methods

Water Changes With Dechlorinated Tap Water

A weekly partial water change of about 20-25% of the aquarium water with dechlorinated tap water can raise the KH of your aquarium water.

This is especially true if your tap water is hard with a higher alkalinity level.

Be careful not to perform the partial water change with ordinary tap water. First, you want to dechlorinate the tap water with a water conditioner or any other chemical-free method. You also want to let the water rest for about 24-48 hours before using it for the water change.

Supplement the Freshwater With Potassium Bicarbonate

Although adding aquarium water with potassium bicarbonate is a way to fertilize aquarium plants, the alkaline mineral also helps in raising the KH levels of the freshwater aquarium.

Potassium bicarbonate is so easy to use as it is a great substitute for baking soda and is also sold in powder form. You want to try out the potassium carbonate in small doses. This will help you get evaluate its effectiveness in raising the KH level.

You can check this potassium bicarbonate dose guide for a start.

Potassium Bicarbonate/Carbonate Dosage Expected Result on KH In Freshwater Aquariums
3.5g of Potassium Bicarbonate per 26.5 gallons of water.Increases KH by 1 dKH
2.5g of Potassium Carbonate per 26.5 gallons of water. Increases KH by 1 dKH

This guide is useful for calculating how much potassium bicarbonate you will need for your aquarium, especially if your aquarium is bigger than a 20-gallon tank.

Use Limestone, Aragonite, or Dolomite As Substrate Sand

You can also raise KH levels by using limestone, aragonite, and dolomite as a substrate in your freshwater aquarium.

Limestone is a sedimentary rock. It is largely made of calcite which is calcium carbonate (CaCO3). Calcium carbonite and other carbonate compounds like magnesium carbonate supply carbonate ions to your freshwater aquariums.

So using limestone as a substrate will naturally help raise KH (carbonate hardness) levels in the fish tank.

Using dolomite rock as a substrate can also help to increase KH levels in the aquarium. Dolomite rock contains dolomite, a double carbonate of magnesium and calcium.

It is also very useful in raising KH levels and improving the alkalinity buffer capacity of the water in your freshwater aquariums when used as a substrate.

As a carbonate mineral, using aragonite as a substrate is another sure way to increase KH levels in your aquariums.

But this method is pretty slow. It can take anywhere from 14 days to about a month and 2 weeks. However, this method is the best if your choice of aquarium fish is hard water fish like the African cichlids.

A great choice of aragonite is the Carib Sea ACS00020.

Add Crushed Coral

Crushed coral is another great choice to increase carbonate hardness and even general hardness over time in your tank.

You can add crushed coral (a few chips) to your substrate or tie the crushed coral in a filter bag and add it to the tank’s filtering system.

Crushed coral is perhaps the healthiest way to raise KH levels in a fish tank because this alkalinity booster increases KH levels steadily.

The acidic compounds that are naturally produced due to the nitrogen cycle in your aquarium will slowly dissolve crushed coral.

This process causes the crushed coral to release calcium and carbonate ions into the aquarium water, raising KH levels.

You can get them at the market at cheap prices. One great recommendation is the Florida Crushed Coral.

Use Alkalinity Buffers

There are commercially available alkalinity buffers specially designed to help you raise the KH of your freshwater aquariums.

These alkalinity buffers are designed to give you flexibility as they can raise both the KH and the pH levels. They are designed to raise pH levels first before increasing KH levels.

If you do not wish to raise your tank’s KH and pH levels, you can opt for alkalinity buffers that maintain pH levels.

Typically, the alkaline buffer will first raise the KH, which will, in turn, raise the pH level, but the release of chemicals will help reduce the pH to bring it to a stable pH level.

You can try the Seachem Alkaline Buffer 600gram.

Use Baking Soda

Baking soda is an easy and fast way to raise KH levels. But it can be risky sometimes because baking soda will increase KH and pH levels together.

And it can raise both water parameters very quickly, causing a sudden change in water condition in the fish tank.

Remember that a rapid change in water parameters such as KH and pH levels can be fatal for your aquarium fish.

You want to be very careful with measuring it. A teaspoon of baking soda can raise the KH of 13 gallons of water by 4dKH!

You also want to remember that baking soda is different from baking powder.

Baking soda consists of alkaline, while baking powder consists of baking soda and many other ingredients that you don’t want in your aquarium.

How To Increase KH Without Raising pH

It is impossible to increase KH levels without raising pH levels.

Every technique used in raising KH level also raise pH levels, and any natural attempt to reduce pH levels will also lead to a decrease in KH levels.

Hobbyists refrain from raising the carbonate hardness level of their freshwater aquariums because they realize how interconnected KH levels are with pH levels.

However, it is safer to maintain the KH and pH of your freshwater tank at levels close to the maximum limit of the recommended ranges.

This is a better alternative to having low KH and pH swings.

Suppose your aquarium water source has a very low KH level, and you keep freshwater fish species that will not tolerate even a slightly high pH level beyond their recommended pH range.

In that case, these unorthodox methods can help you out.

  • Allow your pH value to drop below 6.5. This will naturally happen due to the activities of the nitrifying bacteria in your tank as they break down the fish waste in the fish tank.
  • Put some aragonite chips into the filter of your aquarium (preferably a spoonful or two). It would be best to add the aragonite behind a pre-filter.
  • The aragonite will break down at the same rate that the bacterial nitrification releases acids into the water. This means that the more ammonia, nitrate, and nitrites are released into the water, the faster the aragonite breaks down. The effect is that there will fewer acids affecting the water’s pH level.

It would, however, be best to test the water KH and pH level periodically. This will help you access its effectiveness.

How To Test and Measure KH in an Aquarium

It is important to first check the carbonate hardness of the tap water you use to fill your aquarium.

Measuring the KH of the water in your aquarium is also a significant step in determining whether the aquarium’s KH is sufficient or if it needs to be raised.

You can do this with aquarium test kits, as they usually contain test kits for a number of water parameters.

Using a KH test kit is the most effective way to determine the carbonate hardness of your tank water. These aquarium test kits also help you decide if you need to increase the tank’s KH.

Water testing Recommendations for your freshwater aquariums are as follows:

  • Test once every 30 days if the water’s KH level is 4.5 dKH or more.
  • Test once every 7 days if the water’s KH is less than 4.5 dKH. Keep testing until you have gradually raised it to a value higher than 5 dKH.

A Low KH level is a prompt to pay closer attention to your tank’s water parameters until you have stabilized the carbonate hardness levels. Failure to do so will leave your freshwater tank to risky pH swings and spikes in the nitrite and nitrate levels.

KH (Carbonate Hardness) vs. GH (General Hardness)

KH and GH are often used as though they were perfect synonyms.

You may also find KH and GH confusing because they almost always have the same values. It is usual for hard water to have a high GH level to also have an increased KH level.

Soft water with low GH tends to also have a comparable KH level. But they do not denote the same thing.

While KH and GH are connected to water hardness, it would be inaccurate to use them as though they meant the same thing.

KH and GH are different, and they measure distinguishable parameters in your freshwater aquariums.

Check out this table below to understand how they work.

General Hardness (GH)Carbonate Hardness (KH)
It measures the concentration of dissolved salt in the water (calcium and magnesium specifically).It measures the concentration of dissolved carbonates (CO3) and bicarbonates (HCO3) in the water.
It specifies whether aquarium water is soft or hard. Aquarium fish and plants usually prefer hard or soft water.It functions as a cushion/buffer between the naturally occurring acids in the aquarium water’s pH level.
Matching the aquarium’s water general hardness to the level that aquarium fish is accustomed to can make all the difference between surviving and flourishing.Keeping the KH level within the optimal range will protect the fish tank from sudden fluctuations that result in biological instability.

KH, unlike GH, usually refers to the following, especially when you are trying to change your water level:

  • Carbonate hardness
  • Temporary hardness
  • Acid neutralizing capacity (ANC)
  • Buffer capacity
  • Alkalinity

The Optimal Freshwater Aquarium KH Level

Water parameters fall into different ranges. This is not surprising every fish species prefers water parameters in the ranges similar to their natural habitat. The KH level is no exception.

However, the alkalinity level of your freshwater aquarium will be greatly determined by several factors, key among them being the fish population in the tank.

Other factors include the inclusion of invertebrates in the tank and whether you will be using heavily planted tanks. These factors determine how stable your KH level can be in the fish tank.

However, an optimal range for freshwater aquariums is anything between 4-8 dKH.

While some species, especially the hard water fish, may require a KH level below this range, finding the ideal range of KH level for the particular fish species you are keeping will help you make an informed decision.

There are many freshwater aquarium setups, and while one KH level range may not work for another, this guide below contains some optimal KH ranges for different freshwater aquariums.

It can help you make the best choice for your aquarium fish.


Freshwater Aquarium Type Optimal KH Range Optimal pH Range
Heavily planted fish tank6-12 dKH (for 95% aquarium plant species)6.5-7.5
Tropical fish tank4-8 dKH6.8-7.6
Coldwater fish tank 5-9 dKH 6.8-7.8
Cherry shrimp tank2-4 dKH 6.5-8.0
Discus fish tank1-2 dKH (particularly in breeding tanks)6.0-7.0
African cichlid tank 10-12 dKH 7.8-8.5

Influence of KH on Plant’s Growth in Freshwater Aquarium

Increasing KH in a freshwater aquarium also has great benefits for your aquarium plants.

Knowing how KH affects aquarium plants can help you make good decisions for your aquarium’s ecosystem, especially if you have a heavily planted tank.

Your heavily planted tank is less likely to reach its full potential with extremely low carbonate hardness.

While they may survive, they are less likely to flourish because of the excessive acidity and unstable pH level, typical of low KH levels.

A KH level of 2-7 dKH is usually ideal for a freshwater aquarium. However, whether your aquarium plants require lower or higher KH levels depends on whether they are hard or soft water plants.

Check out this carbonate hardness guide below to know how best to care for the plants in your freshwater aquariums.

Freshwater Aquarium PlantsCarbonate Hardness
Dwarf Baby Tears2-10 dKH
Duckweeds 2-12 dKH
Nymphaea Lotus 2-12 dKH
Anubias 3-8 dKH
Red Leaf Ludwigia 3-8 dKH
Staurogyne Repens 3-10 dKH
Scarlet Temple 4-5 dKH
Ammannia Gracilis4-6 dKH
Monte Carlo Plant 4-10 dKH
Java Moss 5-20 dKH
Beckett’s Water Trumpet 7-20 dKH
Bacopa Caroliniana 2-18 dKH
Christmas Moss 4-5 dKH
Marimo Moss 3-8 dKH
Java Fern 3-8 dKH

Summing Up

Closely monitoring the water chemistry of your freshwater aquariums, especially the KH pH relationship, is a very important step to keeping a healthy aquarium.

Regularly performing water changes and making out the time to measure pH level cannot be overemphasized.

These are the parameters that must be closely watched so that your aquarium fish does not experience serious health issues due to the rapidly changing water condition.

Since water hardness depends on the introduction of alkaline compounds into the tank, increasing the KH level will raise the tank’s pH level.

However, keeping an optimal KH range will reduce the chances of pH fluctuations and pH crashes.

If you wish to raise KH with an alkaline buffer or any of the methods listed above but are scared of raising pH, you can wait until the pH value drops to the minimum of the safe range before you raise KH.

It is important that you measure pH and KH continuously until you have achieved a more stable pH level.

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Kelly Stanley