pH, GH, and KH Explained: Why Water Hardness Matters
pH, GH, and KH confuse many new aquarists, yet together they decide which fish will thrive and how stable your tank stays. This article explains each measure in plain language, how they interact, and why chasing a perfect pH number is usually the wrong goal.
Quick answer
pH measures how acidic or alkaline your water is; GH (general hardness) measures dissolved minerals like calcium and magnesium; KH (carbonate hardness) measures the water's ability to resist pH swings. For most fishkeepers, stable water that suits your species matters far more than hitting a specific pH number.
pH, GH, and KH confuse many new aquarists, yet together they decide which fish will thrive and how stable your tank stays.
pH: acidity, not quality
pH runs from acidic to alkaline on a scale where 7 is neutral. Different fish evolved for different pH: soft-water species like many tetras and gouramis prefer slightly acidic water, while livebearers such as guppies and mollies prefer harder, more alkaline water. Crucially, a stable pH your fish are used to beats a "textbook" pH that swings around.
pH, GH, and KH are separate tests that together describe your water's character.
GH: the minerals fish need
General hardness is the concentration of dissolved minerals, chiefly calcium and magnesium. Fish use these minerals for their gills, bones, and body functions, so GH is not just a number for plants. Soft-water fish are adapted to low GH and can struggle in very hard water, while hard-water fish need those minerals to stay healthy and colourful.
KH: the safety buffer
KH, or carbonate hardness, is the most under-appreciated of the three. It measures carbonate and bicarbonate, which act as a buffer that neutralises the acids constantly produced in a tank. When KH is too low, that buffer runs out and pH can crash overnight, which is genuinely dangerous. A healthy KH keeps pH steady between water changes.
Match fish to your water: soft-water species and hard-water species have very different needs.
How they interact
KH and pH are linked: higher KH generally holds pH higher and steadier. GH and KH often rise and fall together in tap water but are measured separately. This is why simply adding "pH down" chemicals is risky — if your KH is low you may cause a violent swing, and if it is high the pH will bounce straight back. Tap water hardness varies by region, so test your own supply rather than assuming.
Quick FAQs
Should I aim for pH 7 exactly? No. Aim for a stable pH that suits your species; a steady 6.8 or 7.6 is better than a bouncing 7.0.
Does driftwood really change my water? Yes, tannins from driftwood and leaf litter gently lower pH and soften water, which many soft-water fish enjoy.
Can I mix soft-water and hard-water fish? It is usually a compromise for both. It is far better to pick a group of fish suited to the same water.
Will live plants change hardness? Only slightly. Plants mainly consume nitrate and some minerals; they do not dramatically reshape GH or KH.