GH and KH Explained: Hardness in Your Aquarium

Table of Contents

• What Is GH (General Hardness)?
• What Is KH (Carbonate Hardness)?
• The KH-pH Relationship
• How to Test GH and KH
• Singapore Tap Water Values
• Species-Specific GH and KH Needs
• How to Raise GH and KH
• How to Lower GH and KH
• GH, KH and Shrimp Moulting
• Frequently Asked Questions

What Is GH (General Hardness)?

GH, or general hardness, measures the concentration of dissolved calcium and magnesium ions in your aquarium water. These are the two most abundant divalent cations in freshwater and they play essential biological roles for your livestock — calcium is critical for bone development in fish, shell formation in snails and, most importantly, the moulting process in shrimp.

GH is measured in degrees of hardness (dGH) or parts per million (ppm). One degree of GH (1 dGH) equals approximately 17.9 ppm of calcium carbonate equivalent.

What Is KH (Carbonate Hardness)?

KH, or carbonate hardness, measures the concentration of carbonate and bicarbonate ions in the water. While GH tells you about calcium and magnesium, KH tells you about the water’s buffering capacity — its ability to resist changes in pH.

This distinction is important: GH and KH are independent measurements. You can have high GH with low KH, or vice versa, depending on the mineral composition of your water source. They are often discussed together because both relate to water hardness, but they measure fundamentally different things.

KH is also measured in degrees (dKH) or ppm, with 1 dKH equalling approximately 17.9 ppm.

The KH-pH Relationship

KH acts as a pH buffer. Carbonate and bicarbonate ions neutralise acids, preventing the pH from dropping rapidly. This is why KH is sometimes called “buffering capacity.”

How It Works

• High KH (above 4 dKH): pH remains stable and is difficult to lower. The water resists acidification from CO2 injection, tannins, or biological processes.
• Low KH (below 2 dKH): pH can swing significantly, especially overnight when CO2 from fish respiration accumulates without plants photosynthesising to consume it. This can be dangerous for sensitive species.
• KH of 0: The pH has no chemical buffer and can crash to dangerously acidic levels. This is risky unless you are deliberately maintaining a buffered acidic environment using active substrate (like ADA Amazonia, which buffers through a different mechanism).

For CO2-injected tanks, KH is particularly relevant because the relationship between KH, pH and CO2 concentration is predictable. Many hobbyists use the KH-pH-CO2 chart to estimate their dissolved CO2 levels. For a broader look at how all water parameters interact, see our aquarium water parameters guide.

How to Test GH and KH

Liquid drop test kits are the most accurate and widely available method for measuring GH and KH at home.

GH Test

Add one drop of GH reagent at a time to 5 ml of tank water, swirling between each drop. Count the number of drops until the water changes colour (typically from orange/red to green). Each drop equals 1 dGH.

KH Test

The same process applies: add one drop of KH reagent at a time to 5 ml of tank water until the colour changes (typically from blue to yellow). Each drop equals 1 dKH.

Recommended Test Kits

• API GH & KH Test Kit: Widely available in Singapore, affordable and reliable.
• JBL GH and KH tests: European brand, accurate, available from larger aquarium shops.
• Sera GH and KH tests: Another reliable option.

Test strips exist but are significantly less accurate for GH and KH measurements. Invest in a liquid test kit for reliable results.

Singapore Tap Water Values

Singapore’s PUB-treated tap water is consistently soft, which is both an advantage and a consideration for aquarium keepers.

Singapore’s soft tap water is excellent for soft-water species like most tetras, rasboras and many South American cichlids. However, the low KH means pH can be unstable without buffering, and the low GH may be insufficient for shrimp, snails and species that require harder water.

One important note: Singapore tap water contains chloramine (not just chlorine), which is more persistent and requires a quality dechlorinator like Seachem Prime to neutralise effectively.

Species-Specific GH and KH Needs

How to Raise GH and KH

Raising GH

• GH booster products: Seachem Equilibrium is a popular choice. It adds calcium, magnesium and potassium without affecting KH.
• Shrimp-specific remineralisers: SaltyShrimp GH+ (for Caridina) or GH/KH+ (for Neocaridina) are designed to remineralise RO water to precise shrimp-safe parameters.
• Crushed coral: Placed in the filter, it slowly dissolves and raises both GH and KH. Difficult to control precisely but effective for a gradual increase.
• Cuttlebone: A natural calcium source. Break a piece and place it in the tank or filter. Good for supplementing calcium for snails.

Raising KH

• Sodium bicarbonate (baking soda): 1 teaspoon per 40 litres raises KH by approximately 2 dKH. Add gradually and retest.
• Crushed coral or aragonite: Raises both KH and GH. Place in the filter for slow, continuous buffering.
• Commercial KH buffers: Products like Seachem Alkaline Buffer provide controlled KH adjustment.

How to Lower GH and KH

Lowering Both GH and KH

• RO (reverse osmosis) water: RO water has near-zero GH, KH and TDS. Mix it with tap water at the desired ratio to achieve your target parameters. Many serious shrimp keepers in Singapore use 100% RO water and remineralise from scratch.
• Active substrates: ADA Amazonia and similar soils absorb KH from the water and buffer pH to acidic levels. They reduce KH over time but will eventually deplete.

Lowering KH Specifically

• Peat moss or peat granules: Placed in the filter, peat releases tannic and humic acids that consume KH and lower pH. It also tints the water amber.
• Indian almond leaves: Similar effect to peat but milder. Release tannins that gradually lower KH and pH.
• CO2 injection: CO2 reacts with water to form carbonic acid, which consumes KH. In heavily injected tanks, KH can drop noticeably over time.

Since Singapore tap water already has low GH and KH, most local hobbyists are more concerned with raising these values rather than lowering them.

GH, KH and Shrimp Moulting

GH is arguably the most critical water parameter for shrimp health. Shrimp moult regularly as they grow, shedding their old exoskeleton and forming a new one. This process requires adequate dissolved calcium and magnesium — both measured by GH.

Moulting Problems from Incorrect GH

• GH too low: Shrimp cannot harden their new shell after moulting. They remain soft and vulnerable, and death during moulting (failed moult) is common. This is a frequent issue with Singapore tap water used straight without GH supplementation.
• GH too high: The old shell becomes overly rigid and difficult to shed. Shrimp may die trapped in their old exoskeleton.

For Neocaridina shrimp, maintain GH between 6-8 dGH. For Caridina shrimp, 4-6 dGH is ideal. Test your GH regularly and adjust using appropriate remineralisers.

KH matters for shrimp primarily through its effect on pH stability. Caridina shrimp kept on active substrate typically have KH near 0, with the substrate itself providing pH buffering. Neocaridina are more tolerant and do well with KH of 2-5 dGH.

Frequently Asked Questions

Is Singapore tap water too soft for most aquarium fish?

For the majority of popular tropical species — tetras, rasboras, bettas, corydoras, gouramis — Singapore’s soft tap water is actually ideal. These species originate from soft-water habitats in Southeast Asia and South America. You only need to raise GH/KH for hard-water species like livebearers, African cichlids and certain snails, or for shrimp that need consistent calcium levels.

Can I use tap water straight for Neocaridina shrimp?

Singapore tap water is often too soft for Neocaridina shrimp without supplementation. The GH of 2-4 dGH is below the recommended 6-8 dGH range. Add a GH/KH remineraliser like SaltyShrimp GH/KH+ to raise values to a safe level. Also, always dechlorinate — Singapore uses chloramine, which is toxic to shrimp even in small amounts.

Do GH and KH change over time in an established tank?

Yes. KH is consumed by nitrification (the biological filtration process) and by CO2 injection, causing it to drop between water changes. GH can decrease as shrimp and snails absorb calcium and magnesium for shell and exoskeleton formation. Regular water changes replenish these values, and periodic testing ensures they remain within your target range.

What is the difference between GH, KH and TDS?

GH measures specifically calcium and magnesium. KH measures carbonate and bicarbonate buffering. TDS (total dissolved solids) measures everything dissolved in the water — including GH and KH minerals, plus sodium, potassium, nitrate, fertiliser residue and anything else in solution. TDS is the broadest measurement; GH and KH are specific subsets.

Get Your Water Parameters Right

Understanding GH and KH is fundamental to providing stable, species-appropriate water conditions in your aquarium. Singapore hobbyists are fortunate to start with naturally soft tap water — a parameter many overseas hobbyists struggle to achieve — but this softness means supplementation is often needed for shrimp, snails and hard-water species.

If you need help testing or adjusting your water parameters, visit us at Gensou, 5 Everton Park, Singapore. We stock GH/KH test kits, remineralisers and can help you dial in the perfect water chemistry for your specific livestock. With over 20 years of experience, we have helped countless aquascape setups achieve stable, healthy water conditions.

Related Reading

• Aquarium GH vs KH Explained for Beginners: Hardness Made Simple
• How to Raise KH in a Planted Tank Without Raising GH
• Calcium and Magnesium in Aquariums: GH Explained in Detail
• CO2, pH and KH: Understanding the Relationship
• Aquarium KH Crash Prevention: Buffering Soft Water Safely