Aquarium Water Hardness Glossary Deep Guide: GH KH dGH dKH ppm
The phrase aquarium water hardness deep covers two separate dissolved-mineral measurements that hobbyists routinely confuse: general hardness (GH) and carbonate hardness (KH). GH counts calcium and magnesium ions; KH counts carbonate and bicarbonate buffering. One degree of German hardness (1 dGH) equals 17.86 ppm CaCO3 equivalent. The aquarium water hardness deep picture matters because shrimp moulting, plant nutrient uptake and pH stability all depend on it, which is why this guide from Gensou Aquascaping at 5 Everton Park covers conversions, scales and adjustment protocols.
GH and KH Defined Properly
General hardness measures the total concentration of divalent cations, primarily calcium (Ca2+) and magnesium (Mg2+). Carbonate hardness measures bicarbonate (HCO3-) and carbonate (CO3 2-) anions, which together act as the pH buffer. The two are independent. RODI water has zero GH and zero KH; remineralised shrimp water might run GH 6 with KH 0; reef water sits at GH 11+ with KH 8-12.
The Unit Conversions That Matter
1 dGH equals 17.86 ppm CaCO3 equivalent. 1 dKH equals 17.86 ppm CaCO3 equivalent as well, since both use the same scaling. Multiply dGH by 17.86 to get ppm, divide ppm by 17.86 to get dGH. American test kits often report ppm or grains per gallon (1 gpg = 17.1 ppm). European kits report dGH or French degrees (1 fH = 0.56 dGH). Always check which unit your test kit uses before adjusting.
The Freshwater Hardness Scale
Soft water sits at 0-4 dGH (under 70 ppm) and suits caridina shrimp, Amazon tetras, discus and most aquatic plants. Medium runs 5-8 dGH (90-145 ppm) and covers community tanks, neocaridina and Asian biotopes. Hard exceeds 9 dGH (160+ ppm) and supports rift-lake cichlids, livebearers and brackish species. KH targets vary independently — 1-3 dKH for blackwater, 4-6 for community, 8-12 for rift-lake or marine.
Hardness Sources in Your Tank
GH and KH enter the system through tap water, substrate dissolution and intentional dosing. Singapore PUB tap measures GH 2-4 and KH 1-2 — naturally soft and acidic. Aragonite, crushed coral and seiryu stone leach calcium and bicarbonate over time, raising both metrics. ADA Amazonia and similar buffering soils actively bind calcium and lower KH for the first six months.
How to Measure Accurately
Liquid drop kits like the API GH/KH kit count drops to a colour change — each drop equals 1 dGH or 1 dKH. Test kit options at Gensou include the API range and Salifert reef-grade kits with finer 0.2 dKH resolution. Test fresh tap water and aged tank water separately; they often differ by 1-2 dKH due to substrate and CO2 effects.
Raising Hardness for Shrimp and Cichlids
For caridina, use Salty Shrimp GH+ to add calcium and magnesium without bicarbonate, keeping KH near zero. For neocaridina, GH/KH+ raises both. African rift cichlids need crushed coral substrate plus Seachem Cichlid Salt to push GH 9-12 and KH 8-10. Reef tanks rely on two-part calcium/alkalinity dosing through a dosing pump system.
Lowering Hardness Without Stress
Cutting tap water with RODI is the cleanest method — a 50/50 mix halves both GH and KH in one step. Peat moss and Indian almond leaves bind calcium gradually but unpredictably. Avoid acid buffers in soft-water setups — they crash KH and pH simultaneously. Soft-water plant tanks running ADA Amazonia rarely need additional softening.
Why pH Tracks KH
Low KH cannot resist acidification, so pH drifts downward as CO2 from respiration and decay accumulates. KH below 1 dKH risks pH crashes overnight. The opposite is true at KH 10+, where pH locks above 7.8 regardless of CO2 injection — bad news for high-tech planted tanks aiming for 30 ppm CO2.
Singapore Tap Water Practical Notes
PUB tap is soft enough that most community fish thrive without remineralisation. Caridina breeders still RODI-and-rebuild for parameter consistency batch-to-batch. Reef keepers must RODI; tap silicates and phosphates feed nuisance algae. Test your specific block’s water annually — newer estates run slightly different mineral profiles.
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emilynakatani
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