What to Add to RO Water in an Aquarium

A reverse osmosis filtration system provides many advantages to the practice of aquatic husbandry. The multiple stage filter removes chemicals, trace elements and heavy metals from the water that passes through it, which in turn helps to prevent unwanted algae blooms as well as mitigate many other problems related to water quality. The downside of using an RO system to prefilter water is that it radically alters the pH (acid/alkaline balance) of the water. Because of this, additions must be made to the water before introducing it into an established aquatic habitat.

  1. Saltwater Tanks

    • Fortunately for the people who maintain saltwater aquariums most synthetic sea salts contain all of the acidity and alkalinity buffers, in the proper amount needed, when mixed properly. Nearly all synthetic salts such as Instant Ocean and Crystal Sea should be mixed at a ratio of 2 gallons of water to one cup of synthetic salt. This will leave the water with a specific gravity of about 1.023, but always use a hydrometer to test and ensure the proper salinity for your specific aquatic environment.

    Freshwater Tanks

    • Freshwater can be a bit more difficult to fine tune depending on the type of system that you are running. Some freshwater tanks require nothing more than the addition of an acidity buffer to adjust the water to the proper pH level, where other tanks such as cichlid tanks require the addition of an alkalinity buffer as well.

    African Cichlid Tanks

    • For African cichlid tanks, adding a buffer will be required to adjust the alkalinity to the proper level. Use this recipe as an addition to your RO water for the perfect African cichlid all purpose buffer. For every 5 gallons of water, add

      1 tsp. of baking soda
      1 tbs. of epsom salt
      1/2 tsp. of aquarium or marine salt
      You can also use one of the many commercially available buffers from companies such as Seachem or Kent, that are intended for the freshwater environment.

    South American Cichlid and Tropical Tanks

    • For South American cichlids the water chemistry tends more toward brackish than in a typical freshwater aquarium. The buffer mixture for a South American cichlid tank is almost identical to that of the African except for the epsom salt which, in this case, is omitted. So for every 5 gallons of water you will add

      1 tsp. of baking soda
      1/2 tsp. of aquarium or marine salt

      Again, a commercial buffer could be substituted.

      For acid-loving freshwater tropicals, such as Discus, RO water is best left untreated. However, one of the acidifying buffers, using peat moss extractives, can be beneficial to these species when using RO water.