How to Harden Water in an Aquarium

The hardness of water refers to the level of dissolved minerals that it contains. The amount of hardness your water should contain depends on the kind of fish you keep in an aquarium, and can be measured using a water test kit. If the tap water in the area you live in is too soft for your fish, there are some easy ways to harden it. Before doing so, always consult a professional to ensure that your fish will live long and happy lives.

Things You'll Need

  • Crushed coral
  • Limestone
  • Crushed marble
  • Pool salt or appropriate commercial additive
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Instructions

    • 1

      Add limestone or crushed marble into the filter of your aquarium. Calcium carbonate will slowly be released into the aquarium water, raising its hardness.

    • 2

      Put water hardening elements directly into the tank as an alternative to adding them to the filter. Line the bottom of your aquarium with crushed coral instead of regular gravel, or get a limestone rock and put it in the aquarium, but clean it first.

    • 3

      Add pool salt or another water-hardening additive from your fish shop directly into the aquarium's water. Only use additives appropriate for the type of fish you have. Follow the directions on the label for how much to use and how frequently. Add the pool salt or additive each time you change the tank's water.