How to Use Charcoal to Lower the pH in a Fish Tank

Charcoal and activated carbon do not lower pH when added to aquarium water. When used to filter tap water, activated carbon removes the chlorine used to sanitize the water, which will cause a very slight reduction in the water's pH. However, if you add activated carbon or other forms of charcoal to aquarium water, the pH should not change. Instead of charcoal or activated carbon, there are other methods to reduce pH such as peat moss and carbon dioxide. Charcoal can be used in conjunction with these methods.

Things You'll Need

  • Nylon mesh bag
  • Activated carbon
  • Peat moss
  • Can of compressed carbon dioxide
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Instructions

    • 1

      Add about two tablespoons of activated carbon for every 10 gallons of aquarium water. The activated carbon will help remove colors and chemicals from the aquarium water. Change the carbon once every two months.

    • 2

      Add a measured amount of peat moss according to the manufacture's instructions. Peat moss will soften and lower the pH of the water. It will also release tannins that will turn the water yellow, which is desirable in some circumstances such as blackwater aquariums. Activated carbon will remove the tannins.

    • 3

      Set up a air hose to inject carbon dioxide into the aquarium water. Carbon dioxide will lower the pH of the water very well, although you will have to experiment to get the measurements correct. How effective it is depends on the hardness of the water (carbonate hardness or KH), the original pH, how much carbon dioxide you are using and how long the gas is in contact with the water. Start with less carbon dioxide and slowly raise the level until you reach your desired pH.