Aquarium Water Hardness Explained: GH, KH and Why They Matter

Water hardness is one of the most important yet confusing aspects of aquarium chemistry. Understanding the difference between general hardness (GH) and carbonate hardness (KH), and knowing how they affect your fish and plants, is essential for long-term success. This guide on aquarium water hardness explained from Gensou Aquascaping at 5 Everton Park demystifies the topic for Singapore hobbyists.

What Is Water Hardness?

Water hardness refers to the concentration of dissolved minerals, primarily calcium and magnesium, in your water. Hard water contains high mineral levels, while soft water has low mineral levels. In fishkeeping, we measure two types: general hardness (GH) and carbonate hardness (KH). Though related, they measure different things and affect your aquarium in different ways.

General Hardness (GH)

GH measures the total concentration of calcium and magnesium ions. It is expressed in degrees of German hardness (dGH) or parts per million (ppm). Fish, shrimp and snails need calcium and magnesium for biological functions — bone development, muscle function, osmoregulation and shell formation. Soft water species (many tetras, rasboras, Caridina shrimp) prefer GH 2–6 dGH, while hard water species (African cichlids, livebearers) prefer GH 10–20 dGH.

Carbonate Hardness (KH)

KH measures the concentration of carbonate and bicarbonate ions, which act as a pH buffer. Higher KH means more buffering capacity — the water resists pH changes. Low KH makes pH unstable and prone to dangerous crashes, particularly in tanks with CO2 injection or high bioload. A KH of at least 3–4 dKH provides adequate buffering for most setups. African cichlid tanks benefit from KH 8–12 dKH.

The Relationship Between KH and pH

KH directly influences pH stability. In a tank with high KH, pH remains steady even as acids are produced by fish respiration and biological filtration. In a tank with very low KH (below 2 dKH), pH can plummet overnight — a condition called pH crash that can kill fish within hours. If you run a soft water tank, monitor KH regularly and add small amounts of buffer if it drops too low.

Testing Water Hardness

Liquid test kits (API, Salifert, JBL) provide accurate GH and KH readings. The test works by adding drops of reagent to a water sample until a colour change occurs — each drop represents 1 degree of hardness. Test your tap water, your tank water and any water you use for changes. TDS (total dissolved solids) meters provide a quick proxy measurement but do not distinguish between GH, KH and other dissolved substances.

Singapore Tap Water

PUB Singapore’s treated water is generally soft to moderately soft, typically around GH 2–4 dGH and KH 1–3 dKH, with pH around 7.0–7.5. This is suitable for many soft water species without modification. However, species requiring hard, alkaline water (African cichlids, livebearers, brackish fish) need mineral supplementation. The exact values can vary slightly by reservoir source and season, so test periodically.

Raising Hardness

To increase GH, add a GH remineraliser (like Salty Shrimp GH+) or use crushed coral, limestone or aragonite in the filter or substrate. To increase KH, use a KH buffer product or add crushed coral or baking soda (sodium bicarbonate — use cautiously). Seiryu stone naturally raises both GH and KH. For African cichlid tanks, a combination of crushed coral substrate and Malawi buffer salts is the standard approach.

Lowering Hardness

To decrease GH and KH, blend tap water with reverse osmosis (RO) water, distilled water or rain water. RO units remove virtually all minerals, giving you a blank canvas to remineralise to your exact target. Peat moss and Indian almond leaves slightly reduce KH and lower pH through tannin release. Aquasoils (like ADA Amazonia) actively lower pH and KH, which is why they are favoured for soft water planted tanks.

Hardness and Plants

Most aquarium plants adapt to a wide range of hardness, but some have preferences. CO2 injection is more effective in softer, slightly acidic water. Plants that require high CO2 (like HC Cuba, UG, Rotala macrandra) perform best in soft water with GH 3–6 dGH. Conversely, Vallisneria thrives in harder water and may struggle in very soft conditions. Calcium deficiency (from very low GH) causes twisted, stunted new growth in many plant species.

Hardness and Shrimp

Shrimp are particularly sensitive to hardness. Neocaridina (cherry shrimp, etc.) tolerate GH 4–14 dGH. Caridina (crystal shrimp, bee shrimp) require soft water — GH 3–6 dGH with low KH. Insufficient GH causes moulting problems, as shrimp need calcium to form new shells. Too high GH makes moulting difficult as well. For Caridina, most serious breeders use RO water remineralised to exact specifications.

Conclusion

Understanding water hardness transforms you from a passive water-changer to an active water chemist. Know your tap water parameters, match them to your species’ needs and adjust accordingly. Visit Gensou Aquascaping at 5 Everton Park for water testing kits, remineralisers and expert advice on managing water chemistry in Singapore.