FAQs on Reverse Osmosis in a Reef Aquarium

Tapwater can contain things like phosphates, nitrates and heavy metals. In countries like the United States and United Kingdom, regulations require these impurities stay well below the limits safe for human consumption. But what's safe for humans can harm certain aquarium pets.

  1. What is Reverse Osmosis?

    • Reverse osmosis can produce very pure water from tap water. This technique uses pressure to force water through a semi-permeable membrane. The pressure allows water to move "up-gradient," leaving dissolved impurities behind the membrane. This pressure counters water's natural tendency to move toward osmotic equilibrium. The water forced through the membrane has very few dissolved impurities.

    What Types of Fish Need it?

    • Not all fish need water this pure. Many species do fine with dechlorinated tap water. However, some fish come from environments with very little dissolved water, like certain regions of the Amazon. For example, freshwater angelfish (Pterophyllum scalare and P. altum) and discus fish (Symphysodon spp.) need the kind of soft, mineral-free water RO units produce. Additionally, almost all saltwater fish are sensitive to certain dissolved impurities found in some tap water. This makes reverse osmosis water ideal to use with commercial salt mix.

    How Do I Use RO Water?

    • With saltwater fish, RO water only needs some commercial salt mix and it's good to go. However, RO water can cause problems for freshwater fish. RO water is so pure that it has no buffering capability. This makes it vulnerable to large shifts in pH, which can harm fish. High-end pet shops and Internet retailers sell various pH buffers for RO water. These add some minerals to the water, but only enough to buffer it and avoid dangerous pH swings.

    What About Waste?

    • The process of reverse osmosis produces purified water and waste water. The waste water is the water that remains with the dissolved minerals. It has a higher concentration of dissolved minerals than tap water. However, for most applications, other than fish tanks, this difference is trivial. You can use RO waste water for things like washing clothes and watering plants.