Distinguishing between GH (General Hardness) and KH (Carbonate Hardness) can be tricky for novice fishkeepers. A bit of water chemistry insight will give you more control over one of the most critical water parameters: the pH level. This guide will show you how to raise KH in a freshwater aquarium!
To raise KH levels in a fish tank, you need to increase the concentration of dissolved carbonates and bicarbonates in the water. You can do this by adding crushed coral, aragonite, or store-bought alkalinity buffers into the aquarium water.
Carbonate Hardness acts as a blocker between the naturally occurring acids in a fish tank and the pH level. Lower KH leaves pH levels vulnerable to major fluctuations. Higher KH acts as a replenishable buffer between the acids and your tank’s pH level, keeping the latter stable.
The goal of increasing the KH level in a freshwater aquarium is to give the water an improved buffering capacity. This means your pH level will be less likely to fluctuate, ideal for fish health and the tank’s biological balance.
What Is KH (Carbonate Hardness)? Why Does KH Matter In A Freshwater Aquarium?
KH (Carbonate Hardness) is a water quality parameter that measures the concentration of dissolved carbonates (CO3) and bicarbonates (HCO3) in aquarium water.
Why does KH matter in a freshwater aquarium?
KH levels influence the water’s resistance to pH fluctuations.
Acidic compounds (nitrites & nitrates) in aquarium water are natural by-products of the nitrogen cycle. Without the protection of KH, acids in the water can cause dangerous pH fluctuations.
The carbonates and bicarbonates (measured by KH) neutralize acids, buffering their impact on the water’s pH level. Because of this exchange, KH levels in aquarium water aren’t stagnant.
Testing KH levels in a fish tank is usually overlooked until fishkeepers have to deal with the side effects of an unstable pH level.
Low KH, or no dissolved carbonates/bicarbonates at all, can leave your tank’s pH level defenseless.
This can cause water quality to drop to lethal levels for both your fish and live plants.
KH (Carbonate Hardness) vs. GH (General Hardness)
You’ll sometimes see KH and GH wrongfully used interchangeably. This often happens because people have an easier time just using the term “water hardness.” The fact is that KH and GH measure two distinct parameters:
KH – Carbonate Hardness
GH – General Hardness
|Measures the concentration of dissolved carbonates (CO3) and bicarbonates (HCO3) in water.
|Measures the concentration of dissolved salts (magnesium & calcium in particular) in water.
|Acts as a buffer between naturally occurring acids in an aquarium and the water’s pH level.
Keeping KH levels within an ideal range will protect a fish tank against biological instability.
|Influences whether aquarium water is hard or soft. Aquarium fish and live plants can have a preference for one or the other.
Matching the water’s GH to the level that fish are adapted to can make the difference between surviving and thriving.
If you’re researching ways to change your water’s pH level, you’re likely to see KH also being (correctly) referred to as:
- Carbonate Hardness;
- Temporary Hardness;
- Buffer Capacity;
- Acid-Neutralizing Capacity or ANC.
Another reason why GH and KH tend to cause confusion is that they have very similar values in most cases. Hard water, with increased GH, also tends to have high KH levels. Soft water, on the other hand, with lower GH, can have varying KH levels.
Check out more facts on KH vs. GH here:
How To Test And Measure KH In An Aquarium
The most efficient way to test Carbonate Hardness in a freshwater aquarium is to use KH test kits.
You should test your tap water, or the water source you’re using to fill up your fish tanks, first.
Testing tank water itself with KH test kits is easy and will help you determine if you need to increase the water’s alkalinity. If you’re using commercial alkalinity buffers, the test’s reading will also help you accurately dose the buffer.
Carbonate Hardness (KH)
Units of measurement
|Degrees of carbonate hardness (dKH)
Parts per million (ppm)
Equivalent of 1 degree
|1 dKH = 17.86mg of calcium carbonate per liter of water
1 dKH = 17.86 ppm
For freshwater aquariums, it’s recommended that you test the water’s KH levels:
- Once a month if the water’s KH is 4.5 dkH or more;
- Once a week if the water’s KH is under 4.5 dkH until you manage to increase the value above 5dKH.
Low alkalinity should prompt you to monitor your freshwater tank’s water parameters closer until you can stabilize KH levels. That’s because the aquarium will be at risk for dangerous swings in pH, nitrite, and nitrate levels.
What Is The Ideal KH Level For A Freshwater Aquarium?
Like most water parameters, the ideal KH level will fall into different ranges, depending on which fish species you’re keeping. Alkalinity in freshwater aquariums will also be influenced by whether or not the tank is heavily planted.
How densely your tank is stocked, the presence of invertebrates, and other factors can influence how stable KH will be. As a rule of thumb, KH should be between 4-8 dKH in a freshwater aquarium.
Some species, especially those that also prefer hard water, will require the water alkalinity to be below this range. Researching the water parameter preferences of the fish you’re planning to keep (and/or breed) is always recommended!
Here are a few ideal KH ranges for different freshwater aquarium setups:
Type of freshwater aquarium
Ideal KH range
Recommended pH range
|Tropical fish tank
|Coldwater fish tank
|Heavily planted tank
|6-12 dKH (for 95% of aquarium plant species)
|Cherry shrimp tank
|African cichlid tank
|Discus fish tank
|1-2 dKH (especially in breeding tanks)
Why Would You Need To Raise KH In An Aquarium?
An alkalinity level that’s dangerously close to zero in a freshwater aquarium is hazardous.
A pH crash, in this case, is just a breath away. Even KH values that are below 4.5 dKH are a cause for concern. Breeding tanks for soft water fish are an exception!
Unless the fish you’re keeping in a freshwater tank prefer lower GH and KH ranges, allowing alkalinity to dwindle is a dangerous game.
It would help if you viewed Carbonate Hardness as a depletable resource that you sometimes have to supplement.
The more acidic compounds that the carbonates & bicarbonates have to neutralize, the weaker the protective barrier around the water’s pH becomes.
Here are some instances in which it’s advisable to raise KH in a freshwater aquarium:
To avoid a pH crash
You’ve tested the water in your freshwater aquarium, and the KH level is far below the 4-8 dKH recommended range. Big red flag!
Dangerously low alkalinity means any increase in acidic compounds (nitrites, nitrates) can cause pH levels to crash. And these acids are unavoidable, as they’re a natural part of the process of fish waste decomposition.
In this case, raising KH should become a top priority task on your fishkeeping to-do list.
To prevent major pH fluctuations
Allowing KH values to fluctuate between 2-4.5 dKH in a freshwater community tank is just asking for trouble. Especially in an aquarium that’s stocked to a higher capacity (85%), a low KH can cause major pH fluctuations.
A higher KH (above 5 dKH) will result in a more stable pH level. Steady water parameters, be it pH or temperature, are vital to keeping fish healthy, active, and disease-free.
To adjust KH levels to the needs of certain fish species
Tested your tap water, and it turns out municipal water in your area has a particularly low KH?
Raising KH levels might be needed to accommodate the needs of certain freshwater fish. African Cichlids, for example, will do much better in a 10-12 dKH alkalinity range.
Most aquarium fish will adapt to the recommended KH range for freshwater tanks (4-8 dKH).
But if you want to attempt breeding fish that like harder water with increased alkalinity, you might need to use one of the methods listed below.
Remember that changes in water parameters, KH included, should happen gradually. Focus on steadily raising alkalinity into an acceptable range for your fish. It is a healthier approach rather than trying to get a specific reading on a KH test strip.
How To Raise KH In Freshwater Aquariums | Methods
Raising KH in freshwater aquariums increases the concentration of dissolved carbonates & bicarbonates in the water. Fishkeepers have gotten this process broken down into a few basic & efficient solutions:
Use store-bought alkalinity buffers
Commercially sold alkalinity buffers are an excellent aid if you’re looking to increase KH levels in your freshwater aquarium.
Attempting to change the water chemistry of tank water can be intimidating. These buffers are designed to make the process as easy and safe as possible. Just make sure to follow the directions closely!
If both your KH and pH levels are low, use an alkaline buffer that first increases pH, and then raises carbonate hardness. It’s harder to raise the pH after you’ve gotten KH levels up.
Perform water changes using dechlorinated tap water
Are you trying to raise KH from a level that requires weekly monitoring (testing at 4-4.5 dKH) to a more pH stability-friendly degree?
Performing 20-25% water changes using tap water with higher alkalinity could be the solution. If you have particularly hard water on tap, this can replenish your tank’s carbonate hardness easily.
If small KH level tweaks are what your setup needs, you can also try just adding more live plants. They will use up a part of the acids (nitrates) as nutrients, slowing down KH depletion.
Add Potassium Bicarbonate
Supplementing your freshwater aquarium with potassium bicarbonate is a popular live plant fertilizing method. An extra perk is that adding it to your fish tank also helps raise KH levels!
It’s recommended that you use it in small doses at first and test the water to monitor its alkalinity-raising efficiency. Here are some potassium bicarbonate dosing guidelines you can use for reference:
Dose of Potassium Bicarbonate/Carbonate
Expected effect on KH in freshwater aquarium
|3.5g Potassium Bicarbonate per 100 liters water
|Increases KH by 1 dKH
|2.5g Potassium Carbonate per 100 liters water
|Increases KH by 1 dKH
Use Limestone, Dolomite, or Aragonite as Substrate
Adding limestone, dolomite rock, or aragonite in your freshwater aquarium’s substrate mix will help raise the water’s KH level. This is the slowest method to increase carbonate hardness, as the process can last between 2 to 10 weeks.
Any of these rocks can also raise GH (general hardness) in time. This method is more appropriate for freshwater tanks in which you’re keeping hard water-loving fish (African cichlids, for example).
Add Crushed Coral
Crushed coral, especially in a form that’s mixed with aragonite, can quickly raise KH levels. The best thing about this alkalinity buffer supplement is that it changes KH slowly and steadily. The healthiest way a water parameter in a fish tank can change!
Naturally occurring acidic compounds in a freshwater aquarium will dissolve crushed coral slowly. During this process, the coral releases calcium and carbonate ions into the water. This will help raise KH but can also increase general hardness (GH).
You can put coral chips in a substrate mix or tie them in a filter media bag and add it to the tank’s filtering system.
Some fishkeepers shy away from raising KH in their tanks for fear of inadvertently also raising pH levels.
The truth is because Carbonate Hardness is so strongly interconnected with aquarium water’s pH, increasing KH will also raise the pH level.
That being said, stabilizing your freshwater tank’s KH and pH at levels that are near the top of the recommended ranges is safe. It’s actually much safer than having a varying pH level and low KH.
Are you keeping fish that have a strong preference for the lower limit of the recommended pH range for freshwater aquariums?
Are you using tap water with deficient KH levels to fill your tank?
Then your concerns are perfectly valid.
In this case, some aquarists use a less conventional method to raise the tank’s KH:
- Wait for the pH level in your fish tank to drop below 6.5. This will happen naturally, through bacterial nitrification, as fish waste breaks down.
- Add 1-2 teaspoons of aragonite chips into your aquarium’s filter behind any pre-filters.
- Aragonite will dissolve at the same speed as the nitrification process releases acidic compounds into the water. This process tends to work as an automatic neutralization.
- The more ammonia, nitrites, and nitrates being released into the water, the quicker the aragonite chips will dissolve. This means there will be fewer acids eating away at the water’s pH.
Like with any method aimed to raise KH in a freshwater aquarium, test the water periodically while carrying out this chemistry “experiment.”
What Causes KH To Drop In A Freshwater Aquarium?
KH in aquarium water will fluctuate slightly as the carbonates/bicarbonates neutralize acids produced by the fish waste decomposition process. But major fluctuations, and sudden drops in KH, usually occur only in a few instances:
Not doing water changes frequently enough
Carbonate hardness represents the concentration of dissolved carbonates/bicarbonates in aquarium water. This means that it is a resource that’s both depletable and replenishable.
KH gets depleted during bacterial nitrification (the conversion of fish waste into ammonia, nitrites & nitrates). Carbonate hardness should be replenished with periodic water changes.
When water changes aren’t done on a schedule that works for your freshwater tank’s setup, KH levels can drop dangerously low.
Overstocking or overcrowding a fish tank
An overcrowded tank, stocked to more than 85% of its capacity, will have unstable water parameters all-around. KH can drop in this case as the rate at which acids are being produced is accelerated, eating at the water’s carbonates/bicarbonates.
Adding too many new fish into a freshwater aquarium can have the same effect. Once KH levels are too low for pH stability, the tank will eventually go through a pH crash.
Too much CO2
Supplemental CO2 is essential to the growth of some high-demand live aquarium plants, but it becomes dangerous when overdosed. CO2 dissolves into carbonic acid, which turns the water acidic if it doesn’t get used up by plants.
Higher levels of acids in the water will first be buffered by KH, but once KH drops, the pH is next to fall!
KH Influence On Plant Growth In A Freshwater Aquarium
Water alkalinity preferences in live aquarium plants are rarely discussed. But, if you’re looking to create ideal conditions for a heavily planted tank, KH levels should be on your radar as well. Because carbonate hardness is the silent stabilizer of water quality, it’s only natural for it to influence plant growth.
In extremely low KH conditions, excessively acidic water and an unstable pH level can prevent plants from growing to their fullest potential. Although most aquatic plants will survive in low alkalinity, they will be far from thriving or growing vigorously.
In planted freshwater aquariums, a KH level that ranges between 2-7 dKH is acceptable. As you can tell, live plants actually prefer lower alkalinity levels than aquarium fish.
Whether or not the plants you’re keeping are hard water or soft water plants will also influence their preferred alkalinity level. Hard water plants prefer higher alkalinity, while soft water plants prefer lower KH levels.
Freshwater Aquarium Plants:
KH (Carbonate Hardness):
|Dwarf Baby Tears
|Monte Carlo Plant
|Red Leaf Ludwigia
|Beckett’s Water Trumpet
KH (Carbonate Hardness) acts as a protective buffer, increasing water’s resistance to drastic pH fluctuations. Keeping the pH level as stable as possible is vital to fish health & plant growth.
You can use any of the tried & tested methods listed above to raise KH in a freshwater aquarium.
It should be a straightforward process, but there are a few precautions you should take.
Only use one alkalinity boosting method at a time, and test the water’s KH level more often than usual.
This will help you determine if the change is happening at a steady fish-safe pace.
Aim to stabilize your freshwater aquarium’s KH level in a range between 4-8 dKH to keep the tank’s pH level constant.
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