GH and KH for Planted Tanks: What They Mean and How to Set Them

What general hardness (GH) and carbonate hardness (KH) actually measure, why soft-water plants care about KH, sensible target ranges, and how to remineralise RO water.

What GH and KH measure

The 2Hr Aquarist explains that GH (general hardness) is essentially the calcium and magnesium in your water — technically all divalent cations, but calcium and magnesium dominate tap water. KH (carbonate hardness) measures the carbonate and bicarbonate ions, which buffer pH and keep it stable.

Why KH matters for plants and CO2

Many prized 'soft-water' plants are sensitive specifically to KH and tolerate a wide GH range. The 2Hr Aquarist gives a striking example: a tank at 10 dGH but only 1 dKH grows sensitive soft-water species perfectly, whereas a tank at 1 dGH but 10 dKH cannot grow them at all. KH also sits in the CO2 equilibrium, and Tropica notes that at lower pH more bicarbonate is available to plants as usable CO2.

Target ranges

The 2Hr Aquarist runs most of their tanks at around 5 dGH and 0.5 dKH — the low KH suits soft-water fish and sensitive plants, while a moderate GH provides the calcium and magnesium that shrimp and many plants need. This is a good general-purpose target for a planted community tank.

Softening and remineralising

The reliable way to lower hardness is an RO (reverse-osmosis) filter, which strips out essentially all GH and KH through an ultra-fine membrane. That pure RO water must then be remineralised before use to restore calcium and magnesium — a dedicated remineraliser (GH+) does this. Aqua soils also lower KH over time thanks to their peat content.

Sources: The 2Hr Aquarist, PH KH GH and TDS — KH explained, GH explained and Hard vs soft water plants (www.2hraquarist.com www.2hraquarist.com www.2hraquarist.com); Tropica Plant Guide, Fertiliser and CO2 (tropica.com).