How to Use Distilled Water in a Salt Water Fish Tank

Using distilled water can reduce the amount of nitrates, phosphates and other extraneous chemicals found in your fish tank. Distilled water is not pure water, however. Some volatile organic compounds evaporate off at the same temperature as water -- so there will still be some organic materials in the distilled water. Salt water fish tanks get all of their required minerals from the sea salt mixture -- so minerals in the water are not needed.

Things You'll Need

Distilled water
Aquarium sea salt
Hygrometer
Thermometer
Aquarium heater
Gravel siphon
Bucket

Instructions

- 1
  Determine how the distilled water was made and stored. If the distilled water was run through copper condensing tubes, you cannot use it for a marine aquarium. Copper compounds are very toxic to marine corals and fish.
- 2
  Use the hygrometer to determine the current salinity of your tank. Dip the hygrometer into the tank to fill it. Pour off the excess, until the water level is down to the fill line. Read the salinity indicated by the arrow. Water evaporation can increase the salinity over time. You need to keep the salinity between 1.019 and 1.023.
- 3
  Mix the distilled water with the sea salt in a bucket. Use the ratios recommended by the sea salt manufacturer. Most manufacturers recommend a 1/2-cup of salt per gallon of water.
- 4
  Siphon out 20 percent of the old aquarium water. Use a gravel cleaning siphon to clear debris out of the substrate.
- 5
  Check the temperature of the new water. Heat the new water with a spare aquarium heater until it's nearly the same temperature as the aquarium.
- 6
  Add straight distilled water to your aquarium, if the water was too salty. Check the salinity every half-gallon for smaller tanks, and every gallon for larger tanks. The straight distilled water should be warmed to the tank temperature as well.
- 7
  Top off the tank with the fresh salt water.