The new or not so new aquarist may be puzzled by the repeated advice to use purified tap water. So tap water is purified, use that. No.
The water coming out of the tap is purified by the local water authority. Regulations advise the amount of additional substances that are permitted, and, hopefully, the water authority meets that criteria. However, this is for human consumption. We can tolerate an amount of nitrate, phosphate, heavy metal such as copper etc. Obviously we can, we drink it, and clean our teeth with it. Our children do likewise. So what is this need for purification of the tap water for use in a salt water aquarium?
As already mentioned, tap water contains amounts of nitrates, phosphates, and heavy metals etc which are tolerated by humans. The water companies also insert additions such as chlorine and chloramines for cleaning purposes. These additions are not good for the marine aquarium.
The oceans and seas where the livestock of our aquariums originate is stable. The fish and corals have become accustomed to this stability over thousands of years. With some variance, they cannot tolerate much change. For their health and vitality, the aquarist needs to produce the same stability and purity that the livestock is accustomed to. So the introduction of anything into the seawater that will upset the status quo should be avoided. The aquarium is less than miniscule compared to the sea and oceans, and it is fairly easy to upset the balance of things.
Another reason to avoid introducing unwanted additions to the seawater – the sea salt itself. This sea salt is used by the majority of marine aquarists, for the initial fill, and for routine water changes. If the content information on the pack is read, it will be found that there are lots and lots of constituents that make up the salt, some of them present in trace amounts only. The salt manufacturers have taken a lot of trouble to provide, as far as they can, a correct mix that goes a good way to duplicating nature’s own. Then along comes the aquarist and mixes up the salt with tap water, with all the additions that are in the tap water. It makes good sense to mix the carefully proportioned dry salt with water that is as pure as possible, so that minimal changes occur. After all, that is why routine water changes are done, so that the reduction in purity is reversed to an extent by the introduction of new pure seawater.
There is yet another reason why tap water should not be used. The nitrate and phosphate content of tap water varies by location, but it is usually there. Nitrate and phosphate are the culprits in undesirable algae growth, in some cases undesirable algae explosions! No aquarist wants the aquarium in that state. So why introduce it in the first place? Again, have a look at the salt mix packet, it will probably state ‘nitrate and phosphate free’. So again the manufacturers have gone to some trouble on the aquarists behalf. Why negate their help?
The usual way to purify tap water is by using an R/O (reverse osmosis) unit. Put simply, the tap water is forced through a tiny access in a membrane. Pure water is retained and the rest disposed of. The ratio of pure water to the rest is around one in four or five. The purity achieved is from around 95% to 98%.
An R/O unit is rated in gallons per day (occasionally per hour) of purified water, so it is easy to select one that is suitable. They don’t cost a lot, particularly when the cost of livestock and live rock, plus the routine water change sea salt mix, is taken into account.
We’re all trying to furnish the best environment for the life in our aquariums, aren’t we?