Reverse osmosis (R/O) water seems to pop up very regularly in questions and discussions. Some aquarists consider the R/O filter to be another piece of equipment that is not really necessary. There is quite a bit of confusion particularly with beginners, which is understandable when the whole spectrum of equipment and guidelines for keeping marine systems is considered.
The heading question can be answered straightaway – no, you don’t. If you’re lucky, and have a water source such as a well that provides water of high purity, or live in an area where tap water is of a really high standard and not polluted with nitrates, insecticides, metals, bacteria (I won’t go on) then the tank seawater mix could well be of high quality.
Some aquarists have tried using bottled spring water and found that the purity was in fact not as thought and/or the cost was prohibitive. Similarly, some have obtained a supply of distilled water from a chemist or other source and tried that. This proved to be an inefficient way of obtaining the amounts of seawater – multiple gallons – often required, plus the cost.
A few other aquarists have turned to Mother Nature, a fine choice provided the transportation and storage needs can be met, and an unpolluted source can be found.
So for the majority it is the household tap. The water from the mains tap is purified and regulations exist for the amounts of pollutants that are acceptable. Nitrate can be present in a surprisingly high amount, as can phosphate, plus the insecticides etc mentioned earlier.
The water from the tap at the very least should be tested for nitrates and phosphates before consideration is given to using it for a salt mix. If tests are not done then the aquarist is possibly preparing a potential home for nuisance algae, particularly if high power lighting is being used. Added to the pollutants in the tap water are the natural developments in the tank because of the natural processes of the livestock, and the results of potential overfeeding.
Personally, I find it difficult to follow the logic (should I say illogic) of not using R/O water. First, look at the expense the aquarist has accepted in preparing the aquarium system, having purchased lights, salt mix, test kits, a protein skimmer, canister filter, heater(s), circulation pumps, maybe a calcium reactor etc, and also of course the aquarium and stand. Then there is the internal decoration, such as live rock and maybe sand. The fish and corals haven’t been mentioned yet! Whether a fish only or reef system, it all represents a fair sized investment.
Over and over emphasis is placed on the need for high quality water. To this end, the dry salt mix that most aquarists use is manufactured to a high purity, lacking in such unwanted substances as nitrate and phosphate. Why add less than ideal water to it?
If there is a marine retailer close by, R/O water may be available to purchase by the gallon. Quite reasonably, the retailer has to make a profit. Moving the water in containers is not the easiest practice.
If the aquarist purchased an R/O filter then production of purified water would be straightforward and predictable demand easily met. The cost of the filter, particularly when the cost of other items of equipment is considered, is reasonable.
R/O water is generally 95 to 98% pure and the environment for the aquarium livestock would be enhanced, which is probably the most important point, and the possible appearance of undesirable headache algae would be reduced.