This was written by Bob Fenner, regarding natural sea water. Take it as you will.
"The strongest point against real seawater is that it "dies", both biologically and chemically more quickly than synthetics. It's a fact; you must change part of the water more frequently with natural water; depending on the size, type of set-up, filtration, et al. 5-10-20% or more a month is often recommended. Many mixes should be changed just as frequently, but often, especially in terms of appearance (yellowing) you can "cheat" more than with natural water.
Another issue good and bad concerning natural water is that it comes ready equipped with a multitude of micro- and macro-organisms. Even if the water is sediment filtered, diatomed, U-Ved, ozonized, many "things" will survive. What to do then? One or two things: 1) Place the water in a dark place for a couple of weeks before using. 2) Treat the water with copper salts, permanganate, formaldehyde, chlorine, etc. and remove the poisonous effects of the treatment before using. 3) Don't worry; consider the source. Many dealers and hobbyists pour natural water, cold turkey into their systems with impunity. I personally do not endorse item 3). I would treat all natural water as suspect and quarantine and treat accordingly.
There are pro arguments to using real water with little critters or their remains in it to start up a system. One point is that the time needed to establish bio-geo-chemical nutrient cycling (whew!) is decreased greatly. Still another beneficial factor is the ready seeding of the habitat for other microbial needs of the fishes, algaes, invertebrates. Some of the naturally occurring tiny creatures that come in live water are harmful, but most are either beneficial or benign in captive applications.
Natural water should be monitored for pH/alkaline buffering capacity at the very least, and a supply of change water or chemcial preparation be kept close at hand for adjustment. Natural seawater, particularly supplies collected far from shore can exhaust it's buffering capacity quickly (within a day).
The synthetic is in a word, convenient; it serves the purpose as a viable medium for marine life and may be kept on a shelf and almost instantly made ready when need. Despite claims to the contrary, there are little deleterious effects of not pre-mixing, aerating... modern synthetic salt mixes prior to their use. Our corporation's service and retail divisions have used thousands of cases of several brands over the years without trouble for new set-ups as well as routine water changes. If the sea life involved is not otherwise challenged or compromised, you should also have no difficulty."