GH, KH, pH, hard water, soft water — these terms get used interchangeably, but they're measuring genuinely different things. Here's a clear breakdown for freshwater (FW) tanks (saltwater follows similar principles, covered separately).
pH vs. KH (Carbonate Hardness)
pH measures how acidic or alkaline water is (7 = neutral). KH is what actually stabilizes that pH — the water's buffering capacity against carbonates. Reference points: natural seawater runs 7–8 dKH, African cichlid tanks want 8–10 dKH (hard water), while bettas, discus, and many tropicals prefer 3–4 dKH, and caridina shrimp want 0–2 dKH. South Florida tap water runs notably soft, around 1–2 dKH, and drops fast once driftwood or planted substrate is added.
GH (General Hardness)
A separate measurement entirely — the concentration of calcium and magnesium ions (the same minerals behind kettle scale or faucet buildup). GH and KH are independent: you can have water that's hard in GH but soft in KH, or vice versa. Standard test kits usually don't include GH; look for a kit like Aquarium Co-op's that tests both.
Why This Matters for Fish Health
Fish rely on osmotic regulation through their kidneys to stay properly hydrated, and that process depends on a stable KH range. Moving a fish from a KH of 3–4 straight into KH of 1, even after a proper bag acclimation, is a common and under-recognized cause of stress and death — their kidneys can't adjust fast enough. Some species also use KH shifts (like seasonal rain diluting a river system) as a natural spawning trigger.
Adjusting Parameters
Lowering KH/pH: driftwood, or a humic-acid substrate like UNS Controsoil or shrimp soil — these also convert alkalinity into CO2, which is part of why planted tanks benefit from them
Raising KH: an alkaline buffer or crushed coral, especially important in South Florida's naturally low-KH tap water, since nitrifying bacteria stall out below roughly 1 dKH
Raising GH: Seachem Equilibrium or a similar GH booster
Lowering GH: cut water changes with RO/DI water
Any parameter change should happen gradually over 1–2 weeks — fish tolerate rising alkalinity/hardness far better than dropping it quickly.
Testing Rocks and Substrate for Reactivity
Dry a rock, add a few drops of white distilled vinegar, and watch for fizzing — that indicates the rock is carbonate/calcium-reactive and will actively buffer your water even if it looks inert.
Need to adjust your tank's water chemistry? Shop Seachem Equilibrium and alkaline buffers at natureaquariums.com.
