Marine aquarists, particularly those running a reef aquarium, in the main tend to avoid the use of tap water. This is because tap water quality varies quite a lot depending on where the aquarist lives.
In agricultural and/or industrial areas the water may be affected by nitrates, pesticides, heavy metals and the like. In addition bacteria, viruses and spores (example fungal) could be present though these are much more unlikely.
Water authorities have to be sure that the water is safe for human consumption and therefore abide by standards set down for the maximum amount of any substance that can be present. In addition after treatment the water is kept clean by the use of chlorine. Chlorine also helps keep the delivery pipe work clear of bacteria etc. Chloramines are sometimes used on occasion, when a controlled amount is pumped through the pipes for cleansing. In some areas fluoride is mixed into the water as it is said (not without argument) that it is good for teeth.
The tap water may be fine for us humans but we don’t want any of the ‘extras’ in our aquarium seawater. What we want is seawater that is as pure and natural as possible. Dry sea salt is not cheap and it is not logical to pollute the carefully balanced mix with ‘polluted’ tap water.
95%+ pure fresh water from the tap is achieved by using a reverse osmosis (RO) unit. There are other ways but the RO unit is probably the best way for aquarists. Tap water pressure is used to force water through a sediment filter (this is not always present), then a carbon filter where chlorine is removed (chlorine is very detrimental to the next stage), then through a membrane where very nearly only pure water can pass, unwanted substances cannot. Some RO units also incorporate a final cleaning stage using exchange resins, where any final pollutants are removed. This final stage is not absolutely necessary.
So that’s the answer for the aquarist wanting to mix the highest quality seawater, use an RO unit.
Hang on a minute though, there’s usually some kind of disadvantage.
There is and with the RO unit it isn’t really cost as it can be with some equipment. RO units that are well made and functional are available at reasonable prices. It isn’t locating the equipment either as an RO unit can be hidden away in a cupboard close to an incoming water main, placed under the sink, in fact placed anywhere as long as a mains water supply is within reach and there is space available for a collection bucket. There isn’t a cost disadvantage in using the RO unit as with heaters or metal halide lights as they are not electrically driven. Maintenance is usually wide spaced and not a problem (maintenance requirements are subject to the amount of water passed through the unit and the condition of the tap water before treatment).
The disadvantage with the RO unit is linked to performance. This isn’t to say that they are inefficient, far from it, at least as far as the quality of the useable water is concerned. It is the water that is unusable that is the disadvantage.
When an RO unit is connected to the mains water it produces two outputs. One is the purified water (the permeate) that the aquarist collects to use in new salt water mixes and for evaporation top-ups. The other is the unwanted water (the concentrate) that contains all the impurities (solutes) that could not pass through the membrane. The ratio of concentrate to permeate is about 4 to 1. In other words, for every 4 gallons of unusable concentrate there is 1 gallon of useable purified water.
Some aquarists are not too keen on this ‘waste’. However, it is a straightforward exercise to arrange collection of the concentrate in a larger container. The container need not be of high (food) standard as the water inside will not be used for any human or aquarium related purpose. It could be used when needed in a garden for watering, as an example. With a small aquarium the required amount of purified water and therefore waste water may not be seen as a problem, but with a much larger one it depends on the attitude of the aquarist.
The advantages of using purified water from an RO unit for marine aquarium purposes have I think been well demonstrated.