Understanding GH, KH, and pH (and why chasing pH is usually a mistake)

New hobbyists obsess over pH and ignore GH and KH — which is backwards. Let us untangle them.

• GH (general hardness) — dissolved minerals, mainly calcium and magnesium. This is the "hard vs soft water" that actually matters to fish and plants.
• KH (carbonate hardness) — the water's buffering capacity, its resistance to pH swings. KH is the shock absorber for pH.
• pH — how acidic or basic the water is. It mostly follows from your KH and what is in the tank.

Why chasing pH is usually a mistake. People buy "pH down" and fight their water daily. With low KH, pH bounces and can crash — far more dangerous than a stable "wrong" number. Most fish adapt to a stable pH outside their ideal range better than they tolerate a swinging one.

What to actually do:

1. Test your tap water's GH, KH, and pH — let a sample sit 24 hours first, as degassing changes the reading.
2. Stock fish that suit your water rather than re-engineering water to suit the fish.
3. If you must change parameters (e.g., for breeding), do it in a mixing container before the water goes in, slowly.

The glossary defines each term, and the species database lists the ranges fish actually want.

Your turn: post your tap water's GH/KH/pH and what you keep — we will tell you whether to adjust anything or leave it alone (usually the latter).