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How to Lower KH in Aquariums: The Ultimate Guide

By Erika

Keeping the right carbonate hardness (KH) level is essential for a healthy aquarium environment. But what should you do if your KH is too high? In this ultimate guide, we’ll cover everything you need to know about lowering KH in freshwater and saltwater tanks.

You’ll learn why KH matters, methods for reducing it, how to choose the right approach, and step-by-step instructions to safely lower KH. Read on to get the key facts and tips for maintaining ideal carbonate hardness.


KH, or carbonate hardness, refers to the concentration of carbonate and bicarbonate ions in aquarium water. It is one of the most important parameters affecting fish, plants, and invertebrates.

High KH can be problematic in certain setups. It may prevent fish from breeding, inhibit plant growth, raise pH, and interfere with essential chemical processes. That’s why many aquarists find lowering KH is necessary to create ideal habitat conditions.

There are several options for reducing carbonate hardness using natural or chemical methods. The right approach depends on your tank inhabitants, how much you need to decrease KH, and other factors. With some strategic adjustments, you can safely bring KH to the target level.

First let’s review the basics of KH and look for signs your tank water is too hard.

What is KH?

KH is a measure of the buffering capacity of water, or its ability to resist pH changes. Carbonate and bicarbonate ions bind with excess hydrogen ions, preventing the water from becoming too acidic if the pH drops.

Higher KH provides more buffering, making the pH more stable. But extremely hard water can actually create unfavorable conditions for certain fish that prefer soft, acidic environments. It may also limit the bioavailability of essential nutrients and minerals.

Why Lower KH in Some Aquariums?

While ideal KH varies based on the tank inhabitants, values above 9-10 dKH often require lowering. Benefits include:

  • Allows breeding for fish that need soft, acidic water, like tetras, apistos, and rams
  • Promotes growth of plants that prefer low KH
  • Reduces stress for wild caught fish adapted to soft water
  • Increases effectiveness of medications and fertilizers by improving bioavailability
  • Prevents excessive pH spikes that can harm livestock

How to Measure KH

Test kits allow aquarists to measure carbonate hardness. KH test kits use a liquid indicator that changes color based on the concentration of carbonate/bicarbonate ions. Comparator charts convert the color to degrees of KH.

Aquarium stores sell kits that provide accurate KH measurements. Digital meters can also test KH conveniently with automatic reading. Tracking KH helps monitor conditions and effects of KH lowering efforts.

Signs of High KH

Watch for these clues that carbonate hardness may be too high in your system:

  • Inability of fish to initiate breeding behavior
  • Poor growth or death of soft water plants
  • Fish gasping at surface or showing signs of distress
  • pH climbing above ideal range
  • Maximum pH jumps after water changes
  • Ineffective treatments with fertilizers or medications

If you observe any of these issues, test KH and consider options for decreasing carbonate hardness if levels exceed 10 dKH.

Methods for Lowering KH

There are two main approaches for reducing carbonate hardness – natural methods that soften water gradually over time, and chemical methods that work more quickly. Here are some popular options:

1. Natural Methods to Lower KH

Indian Almond Leaves

Derived from a tropical tree, these leaves release tannins that gently acidify water and lower KH. The tannins also offer beneficial antimicrobial properties.

Peat Moss

Peat contains organic acids that soften water. Placing peat moss in the filter slowly releases the acids. KH is lowered gradually over several weeks.

Rift Lake Cichlid Substrate

Specialty substrates contain volcanic materials that leach minerals, creating ideal water chemistry for African cichlids. Using this substrate will help lower KH.

2. Chemical Methods to Lower KH

Acid Buffers

Buffering products blend weak acids that pull carbonate hardness from the water when dosed appropriately. They act rapidly but effects are temporary.

Reverse Osmosis or Deionized Water

These purified waters contain zero KH. Mixing them with tap water effectively dilutes the KH. They provide the purest water for dramatically lowering KH when needed.

How to Choose the Right Method

Selecting the appropriate approach depends on assessing:

  • Tank inhabitants
  • Target KH level
  • How quickly KH needs to decrease

Cichlids and livebearers do well with KH around 5-10 dKH. Tetras, discus and amphibians need very soft water under 5 dKH. Plants vary significantly in their preferred KH range.

Natural methods create a very gradual transition, while chemical approaches bring KH down rapidly. Combining both can allow for gentle long term reduction and precision control.

Consider these factors carefully when deciding on a KH lowering strategy.

Step-by-Step Instructions

Here are detailed steps for using both natural and chemical methods to reduce carbonate hardness:

Natural Methods

Indian Almond Leaves

  1. Add 1-2 leaves per 10 gallons of aquarium water.
  2. Place leaves on substrate, allowing water flow to circulate tannins.
  3. Replace leaves every 2-3 weeks as they decompose.
  4. Monitor KH and repeat until target level is reached.

Peat Moss

  1. Fill mesh filter bags with 2-3 tablespoons of peat moss per 10 gallons of water.
  2. Place a bag in the filter, allowing water flow through the peat.
  3. Replace peat moss every 4-6 weeks as the peat exhausts.
  4. Check KH and continue until the desired level is achieved.

Rift Lake Cichlid Substrate

  1. Purchase a substrate formulated for African rift lake cichlids. Options include crushed coral, aragonite sand, and specialty gravels.
  2. Rinse thoroughly before adding to the tank.
  3. Follow manufacturer instructions for use to optimize water chemistry.
  4. Test KH regularly and perform water changes as needed to facilitate lowering.

Chemical Methods

Acid Buffers

  1. Select an acid buffer product designed to lower KH. Seachem Neutral Regulator, Kent’s R/O Right and others are available.
  2. Before first use, measure tank KH to determine dosage needed to reach target.
  3. At water changes, add buffer according to package directions to reduce KH.
  4. Re-test KH 30-60 minutes after dosing to check adjusted levels.
  5. Use increased dose if further reduction is required.

Reverse Osmosis or Deionized Water

  1. Obtain an RO or DI filtration unit to produce pure water with 0 KH.
  2. When making water for changes, blend the RO/DI water with tap water to achieve the desired KH dilution.
  3. Start with a mix of 75% RO/DI water to 25% tap water and test KH.
  4. Adjust the ratio as needed to incrementally reduce KH with each water change.
  5. Eventually transition to 100% RO/DI water changes once target KH is reached.

Monitoring KH Levels

Consistently testing KH is crucial to make sure your efforts are succeeding in lowering carbonate hardness within the desired range. Here are some tips:

  • Use an aquarium KH test kit at least twice per week while actively lowering KH. This allows you to accurately track changes.
  • Make adjustments incrementally if KH drops too rapidly. For example, use a lower dose of buffer or less RO/DI water in the mix.
  • When maintaining lower KH levels, continue testing 1-2 times per month. This ensures KH remains stable long term.
  • Keep KH within the ideal range for your tank inhabitants. Some periodic buffering may be required for maintenance.

With routine monitoring, you can safely bring KH to healthy levels for your fish, plants and invertebrates.


Lowering high KH is often necessary to provide optimal water chemistry for soft water fish, sensitive plants, breeding setups, and natural biotope aquariums. There are many effective options for reducing carbonate hardness using natural substances or chemical buffers.

The right choice depends on the tank inhabitants and how much you need to decrease KH. Indian almond leaves, peat moss and specialty substrates gradually soften water over time. Acidic buffers or purified RO/DI water will lower KH rapidly when quick results are needed.

Regular monitoring allows you to fine-tune the KH reduction process. With some strategic adjustments, you can successfully maintain carbonate hardness at the perfect level for a thriving freshwater or saltwater aquarium.

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