Water Hardness for Planted Tanks: GH and KH Explained Simply

GH measures general hardness, the calcium and magnesium in your water. KH measures carbonate hardness, the buffer that keeps your pH from swinging. For a planted tank, stability matters far more than hitting one perfect number.

GH: the minerals plants and livestock use

Calcium and magnesium are nutrients. Stem plants pull them from the water column, and shrimp and snails need them to build shells and molt. Most community plants sit happily around 4 to 8 dGH.

Soft water below 3 dGH can leave plants deficient and shrimp struggling to molt. If you keep inverts, GH is the number to watch, as our guide on neocaridina shrimp care explains in more detail.

KH: the buffer behind stable pH

KH is your safety net. Carbonates neutralize the acids that build up from fish waste and CO2, so a tank with some KH holds a steady pH. A tank near zero KH can crash overnight.

Aim for 3 to 6 dKH in a typical planted tank. If you inject CO2, a little KH keeps the daily pH shift gentle instead of jarring.

How substrate and dosing change your numbers

Active aquasoils intentionally lower KH and GH to soften water for demanding plants and Caridina shrimp. That is a feature, not a fault, and our breakdown of the best planted substrate covers which ones buffer and which stay inert.

Your fertilizer plays a role too. Some routines add magnesium and calcium directly, which nudges GH up. If you dose heavily, the schedule in our EI dosing guide shows where those minerals come from.

Raising or lowering each one

To raise GH, add a remineralizer made for planted or shrimp tanks. To raise KH, crushed coral or a small dose of baking soda works. To lower both, cut your tap water with RO or rainwater until the numbers land where you want.

Change values slowly. A sudden shift stresses fish and shrimp more than water that was a little off to begin with.