GH and KH Explained: Why Your Freshwater Plants and Fish Care

Two different things people often confuse

GH (General Hardness) measures total dissolved calcium and magnesium — the minerals that matter for fish osmoregulation, plant nutrient uptake, and shell-building invertebrates like snails.

KH (Carbonate Hardness), also called alkalinity in freshwater contexts, measures buffering capacity against pH swings — it's the parameter that determines how stable your pH stays day to day.

Why species-specific targets exist

Soft-water fish (many tetras, rasboras, wild bettas) evolved in low-mineral blackwater environments and struggle with high GH. Hard-water species (most African cichlids, livebearers) need mineral-rich water to thrive and breed successfully. Mismatching either direction creates chronic, low-grade stress that shows up as poor coloration, reduced breeding success, or unexplained lethargy.

The practical fix

If your tap water doesn't match your livestock's needs, RO/DI water remineralized to target GH/KH gives you full control rather than fighting your local tap chemistry. For most community tanks, matching the middle of your species' tolerance range — not the extreme — gives the widest margin for error across a mixed stocking list.

Test GH and KH together, since a low KH with no buffer is the most common cause of unexplained pH crashes in freshwater tanks running RO water without remineralization.