How to Use an RO System in a Fish Tank

RO, or reverse osmosis water, is favored by aquarium enthusiasts because it is extremely pure. Regular tap water can contain many chemicals, harmless to humans but potentially toxic to fish, such as small amounts of chlorine. Since RO water has so few chemicals in it, it is easier to alter the water chemistry compared with tap water, in which dissolved minerals may act as a buffer.

Things You'll Need

  • 5 gallon bucket
  • Thermometer (digital is best, liquid crystal is acceptable)
  • Submersible aquarium heater
  • Hydrometer
  • Aquarium salt
  • Water conditioners (pH, trace minerals)
  • Water test kits (pH, trace minerals)
  • Plate (optional)
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Instructions

    • 1

      Heat the water to the temperature of your aquarium. This is important for two reasons: It can shock your cold-blooded fish if there is a sudden change of temperature, and most water conditioners should be added at the same temperature as the aquarium.

    • 2

      Blend with tap water. This is an optional step for freshwater setups only. You should not use 100 percent pure RO water as it can actually be more pure than the water to which the fish are adapted. A 10-to-1 ratio of RO to tap water will work in most situations.

    • 3

      Adjust the pH using commercially available water conditioners. Once again, this is an optional step that depends on what species you are keeping.

    • 4

      Add salt or commercially available trace minerals. For freshwater setups, just add the trace minerals per the manufacturer's instructions. For saltwater setups, carefully add salt, using your hydrometer to measure the salinity.

    • 5

      Pour the prepped water into the fish tank. If you have decorations or live plants/corals, consider placing a plate on the substrate and gently pouring the water onto that so as not to disrupt your tank.