Reverse osmosis is a way of making the water really pure and bring the hardness down to non detectable levels. RO water is for instance used by aquarists who want to breed Apistogramma cichlids, since these fishes normally need really pure and soft water to reproduce successfully.
Before you start tinkering with the water chemistry in your aquarium, there are several things to keep in mind. To begin with, many fish species hate rapid changes. A rapid change from poor water conditions to ideal water conditions can actually be more dangerous than keeping the fish in poor water conditions. There is however plenty of exceptions to this rule and a major water change that dramatically changes the conditions in the aquarium is known to induce breeding in many species. Before you make any changes in your aquarium you should therefore research your particular species to find out if they need a slow and gradual change or if they can cope with dramatic alterations in water chemistry.
Secondly, mimicking the natural conditions of a fish is not always a good idea. If you purchase specimens that have been bred and raised for many generations in your area, they may have adjusted to the local tap water and will happily live, spawn and raise their young in such conditions. Fiddling with the water chemistry will then be a waste of your time, money and energy, and it can even be dangerous for your fish.
Thirdly, fish should never be kept in pure RO water. You can read more about this further down in this article.
What is osmosis and what is reverse osmosis?
When two water volumes (or other solvents) are separated by a semi permeable membrane that blocks the transport of solutes, water will flow from the side of low solute concentration to the side of high solute concentration. If you apply external pressure on the side of higher concentration, you can stop the flow. If you apply even stronger external pressure, the flow will be reversed – i.e. you will have created reverse osmosis.
Getting an RO unit
Before getting an RO unit, ask yourself how many aquariums you plan to keep simultaneously, how big they are and how often you will change the water. If you for instance have two 100 gallon aquariums that you change 50% of the water in once a week, a big 50 gallon a day RO unit will be a waste of money. If you on the other hand believe that you will keep getting more and more aquariums that need RO water (many aquarists do), then getting a major RO unit from the beginning may save you money in the long run.
Never use pure RO water
As mentioned above, fish should never be kept in pure RO water with a 0 ppm of total dissolved solids (TDS) since such extremely pure water is not what they are used to from the wild. You can purchase chemicals from your local fish store and use them to add all the necessary ingredients that will make the RO water suitable for fish, but this can be quite expensive, especially over time. A cheaper way of turning pure RO water into usable RO water is simply to add some of your normal tap water or the very hard “waste water” that will be produced by the RO unit. Use your test kit to find out the ideal blend.
Adjusting the pH-value
Many fish species appreciates and addition of peat moss to the RO water since it will keep the pH-value around 5.5 and make the water tea colored – just as in their native habitat. You can either filter the water through the peat moss or let peat moss rest in a bucket of RO water for a week or two before using the water.
