Can Fish Live in Tap Water if It Has a Water Softener?

Different species of fish have adapted to a shocking variety of water conditions. However, most fish have a preferred set of water parameters that mimic their home waters. Many species need soft water. However, water softeners soften water in a way that makes it unsuitable for aquarium fish.

  1. What Is Hardness?

    • The term "hardness" refers to water's ability to precipitate certain mineral salts, including calcium bicarbonate and magnesium. Hard water tends to leave mineral stains containing these substances when it evaporates. In homes, hard water makes it hard to do chores like the laundry, since hard water has a harder time dissolving soap. Many fish come from habitats that naturally have hard water. However, those that do not get stressed and are more prone to disease when exposed to these conditions.

    What Water Softeners Do

    • Commercial water softeners usually run water through clays or resins that absorb calcium and magnesium with sodium salts. This makes life easier for household chores. However, this does not improve water quality from an aquarium hobbyist's standpoint. In the wild, fish would probably not encounter water with sodium salts but little dissolved calcium and magnesium. This unusual combination of minerals makes the water unsuitable for both fish that prefer hard water and fish that prefer soft water.

    Reverse Osmosis

    • The process of reverse osmosis can soften water for aquarium use. In reverse osmosis water is forced through a membrane that allows water through, but not minerals. This means that water run through an RO unit is free of all minerals, unlike water run through a home water softener which just "trades" one set of minerals for another. This produces water that is perfect for species that come from soft water.

    Which Fish Need Soft Water

    • Generally, fish from rain forest streams and lakes need soft, acidic water. This includes fish like South American cichlids and tetras. However, some fish come from water that is naturally hard and rich in dissolved minerals. This includes African cichlids and fish from Central America. Salt water usually has plenty of dissolved calcium and other minerals. However, you should always prepare water for a marine aquarium by mixing commercial salt mix with RO water. Salt mixes have all the minerals marine organisms need and work best with RO water, since it is chemically a blank slate.