Q. I have heard so much about using reverse osmosis (RO) water in reef tanks, but I can’t afford an expensive RO unit. I was wondering if I could use dehumidifier water as a cheap substitute. I know it has no minerals in it, but supplements take care of that for me. What advantages does RO water have over regular tap or dehumidifier water?
A. I once wondered the same thing as I sat in my basement and saw the amount of water my dehumidifier would collect in the summer months. After thinking about it for some time I came to the conclusion that using this water in my reef tank may not be such a good idea, and I’ll tell you why.
In the summer months you usually get a lot more water being produced than in the drier winter months. Conversely, your aquarium probably has a greater evaporation rate in the winter than in the summer. Therefore, depending on the evaporation rate of the aquarium and the amount of humidity in the air, you may or may not be able to generate enough water to meet the demand. This, of course, varies depending on where you live and where the aquarium is located in your home.
I was also unsure as to what the coils in the dehumidifier were made of. It might be possible that water forming on the coils could pick up trace amounts of aluminum, zinc or copper. And, as the water condenses it could absorb compounds from the surrounding air, such as nicotine from cigarette smoke, compounds from household cleaning products and so on.
The final thing that made me choose against using this water was the color of the plastic container the water collected in — it gradually became a rusty orange color! I decided that the money and time I had invested in my reef tank was not worth risking on a questionable water source.
Reverse osmosis units can be purchased for under $200 and may cost as little as a good dehumidifier. So why take the risk? If you are really intent on trying this, I would suggest having the water checked for organic compounds and metals, but the cost of having these tests done will most likely be greater than buying an RO unit. I would also purchase a deionizing column and pump the dehumidifier water through it first to remove any possible contaminants before using it.
RO units have several advantages over tap water and dehumidifier water. The water comes from the tap, which is usually safe and is filtered by a fine sediment filter and carbon block before it enters the RO chamber. Once it passes through the membrane, many units also pass the water through one or more deionizing units. This results in very pure water that is low in organics, dissolved minerals and nutrient compounds. If you live in an area with tap water that is very low in phosphate and nitrate and high in calcium and magnesium, you may be able to use this water directly. However, most city water treatment plants regularly add phosphate to the water under federal guidelines, so it is usually safe to assume that your tap water will contain phosphate.