How to Soften Hard Water in Aquariums

Written by naomi bolton Google
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How to Soften Hard Water in Aquariums
Angel fish require soft aquarium water. (angel fish image by Christian Schoettler from

Hardness indicates the amount of dissolved salts that water contains. These mineral salts include carbonates, bicarbonates, chlorides and sulphates. Fish species that inhabit slow-moving rainforest rivers are accustomed to soft water, which lacks mineral salts, and will require water with a similar chemistry when placed in an aquarium. Popular aquarium fish types from the Amazon, such as discus, Oscars and angels, all require particularly soft water. Aquarists can re-create these natural water parameters by removing mineral salts from tap water or by collecting soft water. Many options exist, ranging from passing water through a reverse osmosis unit to adding natural peat extracts.

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Things you need

  • Large plastic drum
  • Water hardness test kit
  • Reverse osmosis unit
  • Ion exchange unit
  • Natural peat extract
  • Driftwood
  • Bottled water

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  1. 1

    Collect rain water by placing a plastic drum under a drain pipe. Ensure that all guttering and down pipes are plastic and that they have been previously cleaned.

  2. 2

    Collect rain water only in areas that do not experience high levels of atmospheric pollution. Wait a few minutes before placing your drum under the drain pipe. This allows dust and detritus to be washed away before you begin collecting.

  3. 3

    Mix tap water slowly with the rain water, until you have the required hardness. Use a hardness test kit to gauge the hardness. Gauge the hardness of your rain and tap water mixture by taking a sample of water and adding the required number of drops of reagent. Compare the colour of your sample with the hardness colour chart. The number of drops that you will require will depend on the make of test kit.

  4. 4

    Run your tap water through a reverse osmosis unit. The semi-permeable membrane in these units only allows pure water molecules through. Those salts that raise the water hardness are removed from the water that is available to use.

  5. 5

    Pass the water over resin beads. This process, termed ion exchange, provides soft water by removing mineral salts from the water. Ionised salt molecules become attached to the resin beads and are thereby removed from the water.

  6. 6

    Add natural peat extracts, which contain trace elements and humic acids, to your filter. Aquarists also replace water that has evaporated from the aquarium, with water in which peat has been allowed to soak.

  7. 7

    Include pieces of driftwood in your aquarium. Driftwood releases tannins into the water, which will assist in lowering the hardness and in maintaining an already low hardness. To derive the maximum benefit from driftwood, add as many large pieces as possible.

  8. 8

    Use bottled water, particularly when only small volumes are required, such as for top-up purposes.

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