Understanding Water Hardness (GH)

    Beyond pH lies General Hardness—the essential minerals that power your fish's biological functions.

    What is General Hardness?

    Aim for roughly 0–6 dGH for soft-water species, 7–12 dGH for a general community tank, or 13+ dGH for hard-water species (most freshwater shrimp do best around 4–8 dGH). General Hardness (GH) refers to the amount of dissolved Calcium (Ca2+) and Magnesium (Mg2+) ions in your water.

    Think of minerals as "electrolytes" for your fish. Without the correct GH levels, fish cannot properly regulate their internal salt balance (osmoregulation), leading to severe stress or "soft-shell" issues in invertebrates.

    Soft (0 dGH)Moderate (10)Very Hard (25+)
    Soft Water (0 - 6 dGH)
    Low mineral content. Mimics soft rain-fed rivers and blackwater environments.
    Typical Inhabitants: Discus, Neon Tetras, Rams, Bettas
    Moderate / Transition (7 - 12 dGH)
    A balanced mineral profile suitable for the widest variety of tropical community fish.
    Typical Inhabitants: Angelfish, Corydoras, Barbs, many Plants
    Hard Water (13 - 30+ dGH)
    High in Calcium and Magnesium. Essential for bone growth and osmoregulation in specific species.
    Typical Inhabitants: Guppies, Mollies, African Cichlids, Goldfish

    Why Hardness is Vital

    Cellular Function

    Calcium is necessary for muscle contraction, bone development, and the transmission of nerve impulses in fish.

    Invertebrate Care

    Shrimp and snails require Calcium to build their shells. Low GH results in "The White Ring of Death" or shell erosion.

    Plant Growth

    Magnesium is the core atom of chlorophyll. Without adequate GH, aquatic plants turn yellow and stop growing.

    GH vs. KH — What's the Difference?

    General Hardness (GH)

    Dissolved calcium and magnesium — minerals your fish, shrimp and plants use directly for cell function, shell-building and growth.

    Carbonate Hardness (KH)

    Carbonates and bicarbonates — your tank's pH buffer, not a mineral your fish absorb. See the Alkalinity (KH) Guide for the full picture.

    dGH to ppm Conversion

    1 dGH ≈ 17.9 ppm (mg/L). Most liquid test kits read in degrees (dGH), while some strips read directly in ppm — check which unit yours uses before comparing it to a target range.

    GH for Shrimp Tanks

    Shrimp pull calcium from the water to build a new exoskeleton every time they molt. Most freshwater shrimp do best around 4–8 dGH; too low, and molts fail or shells go soft.

    Practical Tips for Managing Water Hardness

    1

    GH vs KH

    GH measures Calcium/Magnesium (for fish health). KH measures Carbonates (for pH stability). Both are vital.

    2

    Evaporation Increases Hardness

    As water evaporates, minerals stay behind. Regular water changes are the only way to export these minerals.

    3

    Natural Hardening

    Add crushed coral, Texas Hole Rock, or limestone to your filter to slowly increase mineral content.

    4

    Softening Water

    The most effective way to lower GH is using an RO (Reverse Osmosis) unit or mixing in distilled water.

    Note for Soft-Water Keepers

    While driftwood and leaves lower pH, they do not effectively lower GH. To reduce hardness, you must physically remove the minerals using an RO system or by diluting with rainwater/distilled water.

    Frequently Asked Questions

    GH stands for General Hardness, which measures the concentration of dissolved Calcium and Magnesium ions in the water. It is measured in degrees (dGH) or ppm.

    It depends on the species. South American fish like Discus prefer soft water (low GH), while Central American livebearers and African Cichlids require hard water (high GH) for survival.

    To lower GH, you must dilute the minerals in your tank by performing a water change with Reverse Osmosis (RO) or distilled water. Driftwood and peat moss do not significantly lower GH.

    GH (general hardness) measures dissolved calcium and magnesium — mineral content your fish and plants use directly. KH (carbonate hardness) measures your water's buffering capacity, which stabilizes pH. A tank can be high in one and low in the other.

    Multiply dGH by roughly 17.9 to get ppm (mg/L). Most liquid test kits read in degrees (dGH), while some strips read directly in ppm — check which unit your kit uses before comparing it to a target range.

    Related water chemistry guides

    GH is one piece of overall water chemistry — these cover the rest.