GH, KH and pH: understanding and adjusting aquarium water hardness
Beyond ammonia and nitrite, water hardness is a parameter beginners often overlook, even though it directly affects the wellbeing of many species. Here's how to read and adjust GH, KH and pH without putting fish at risk. The care and grooming category gathers all our maintenance guides.
What does GH measure?
GH, or general hardness, reflects the amount of dissolved calcium and magnesium in the water. Soft water (low GH) suits Amazonian species like neon tetras, while hard water (high GH) is preferred by Lake Malawi cichlids or certain brackish-water fish.
What does KH measure, and why does it matter so much?
KH, or carbonate hardness, measures the water's ability to buffer pH swings. A KH that's too low leaves the tank exposed to sudden, dangerous pH crashes, particularly as organic acids build up between water changes; adequate KH, by contrast, keeps pH stable over time.
How do you adjust these parameters gradually?
- Lowering GH/KH: dilute with reverse osmosis water, blending it gradually with tap water rather than a sudden switch.
- Raising GH/KH: add a calcareous substrate or a dedicated remineralizer, in small repeated doses.
- Correcting pH directly: generally best avoided, since an unstable pH is more dangerous than one that's slightly off-target but steady.
Why does stability matter more than the exact value?
A landmark review by Evans, Piermarini and Choe, published in 2005 in Physiological Reviews, details the central role of gills in fish ion and acid-base regulation, a system already working continuously to maintain internal balance against the surrounding water (study on Google Scholar). Rapid swings in GH or pH place extra strain on an organ that's already under constant load, which is why gradual adjustment beats a sudden correction, as covered in our guide on water tests.
Frequently asked questions
Do you need to adjust GH/KH for every fish?
No, most common species tolerate a wide range; adjustment mainly matters for demanding species or breeding projects.
Does KH affect plants directly?
Indirectly, yes, since it shapes carbon availability, a topic covered in our guide on fertilizer and CO2.
How often should you test GH and KH?
Once a month is enough for routine monitoring, plus after any change of water source or remineralizer dose. Browse all our guides on the fish hub.
This guide is part of Planète Pets’s Fishkeeping universe. Our advice is general in nature: for any health concern, your veterinarian remains the only reference.