Low KH in a freshwater aquarium: how to raise it
Symptom: Your KH test reads 2 dKH (about 36 ppm) or less, or pH keeps dropping.
Likely causes, ranked
1. Buffer used up between water changes
very commonDiagnosticKH drifts down week by week; pH follows it; water changes are infrequent.
FixChange water more often with tap water that has some KH, and add a buffering substrate. NRAC notes unsupplemented recirculating systems lose alkalinity steadily, while ponds usually hold it.
2. Soft source water or pure RO water
commonDiagnosticTap or RO water tests near 0 dKH.
FixAdd buffer: Aquarium Co-Op suggests about 1 lb of crushed coral per 10 gallons, replaced every 6–12 months as it dissolves; NRAC recommends sodium bicarbonate for raising alkalinity in recirculating systems. Raise it gradually, testing as you go.
Common myths
ClaimKH and GH are the same thing.
RealityAquarium Co-Op and NRAC both separate them: KH (alkalinity) is carbonate and bicarbonate; GH (hardness) is calcium and magnesium. They often run together but can differ widely.
Questions
What KH should a freshwater aquarium have?
Aquarium Co-Op recommends 4–8 dKH (70–140 ppm) for most tanks and warns against 2 dKH or below.
How do I raise KH quickly?
Sodium bicarbonate raises alkalinity (NRAC), but raise it gradually; crushed coral raises it slowly and keeps it up.
Can low KH stall my cycle?
Yes: UF/IFAS says nitrifying bacteria can't function if alkalinity is below 20 mg/L.
How do I convert ppm to dKH?
1 dKH is about 17.9 ppm as calcium carbonate (Aquarium Co-Op); our alkalinity converter does it for you.
Related
Sources & references
- 1.Aquarium Co-Op — The fish keeper's guide to pH, GH and KH (retailer guide)https://aquariumcoop.com/blogs/aquarium/ph-gh-kh
- 2.Buttner, Soderberg & Terlizzi — An introduction to water chemistry in freshwater aquaculture, NRAC Fact Sheet 170 (1993)https://southcenters.osu.edu/sites/southc/files/site-library/site-images/WaterChemistryInAquacultureNCRAC.pdf
- 3.Francis-Floyd, Watson, Petty & Pouder — Ammonia in aquatic systems, UF/IFAS FA16 (2009)https://extension.rwfm.tamu.edu/wp-content/uploads/sites/8/2013/09/Ammonia-in-Aquatic-Systems.pdf
Last reviewed