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Related FAQs: Seawater, Seawater 2, Seawater 3, Seawater 4, Seawater 5, Seawater 6, & FAQs on Mixing, Supplementing, Storing, Moving, Physical/Chemical Troubleshooting/Fixing... By Make/Manufacturer: Natural Seawater. SyntheticsAquarium Systems (Instant Ocean, Reef Crystals), Aquacraft (Marine Environments, BioSea...), Central Garden (Oceanic), Kent Marine (SeaSalt), Red Sea (Red Sea Salt, Coral Pro Salt), SeaChem Marine (Marine Salt, Reef Salt), Energy Savers (Coralife), Tropic Marin, Other Brands... About Buying Pre-mixed Seawater, About  Synthetics Manufacturers Advertising Claims...  Spg 1, Treating Tapwater For Marine Aquarium Use, Reverse Osmosis Filtration

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/The Conscientious Marine Aquarist

Sea Water, Natural or Synthetic?

(Which way to go?)

By Bob Fenner

Pre-Mixed Seawater Set-up

Every few years a few standard "urban myths" (e.g. to float new arrivals or not), seem to re pop-up in our hobby. One of my favorite, and most disturbing is the issue of "real" seawater versus synthetic mixes. Here I'm referring to legitimate formula-tions as opposed to "Wonder Water", "Magic Ocean", and other sugar-based let's-raise-the-specific-gravity-gravity-without-increasing-the-ionic-content mixes (supposedly allowing the succesful co-existence of marine and freshwater organisms). Nor do I mean to include "lobster" system water softener grade formulations in this discussion. I mean here to disparage the claims of the purveyors of so-called "natural" seawater. These assertions also apply for areas in the world with easy access to the oceans where there arises the choice for the marine aquarist to use a saltwater mix or natural water. There are, admittedly, many valid arguments pro and con for either alternative.


The better mixes can retail for 30-40 cents per gallon, or more depending on how much you buy. "Live" ocean water costs the price of time, travel and proper filtration for you to collect and process it, or typically somewhere @$1.00-$1.50 retail per gallon to purchase.

In San Diego, free, sand-filtered seawater is available usually 24 hours a day at the base of the pier at Scripps Institute of Oceanography, U.C.S.D., La Jolla. This service is available at many other coastal towns. If you are dealing with large volumes of water the dollar savings can be considerable in treating and using this water, but it does have it's drawbacks:


More vital than it's expense is the water's relative ability to support marine life. Most of the more complete mixes are capable of sustaining marines for extended periods of time. The history of their use is impressive; they have been used all over the world by science, public aquaria and hobbyists alike for decades. The best available salt mixes have been demonstrated to support many invertebrates and fishes that live in close conjunction with them without further additives or modification.

Natural water that is pH/alkaline reserve checked and, if necessary, adjusted will support all forms of marine life.


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.

Trace Elements:  

Are a particularly contentious, confusing issue that continues (to my bafflement) to perplex & impress aquarists. Edmund Mowka (1979, 1980) presents a concise, lucid treatment of the subject. The long and short of it is it appears that most so-called "trace" elements and compounds are:

1) Just that. Small, often transient materials of no or miniscule biological consequence.

2) Readily lost by physical, chemical and/or biological mechanisms in captive systems.

3) Much of the "trace" benefit is derived from food/nutritive sources, not from water per se.

4) For "reef" systems with substantial amounts of invertebrate and intentional algae matter, must have chemical supplementation whether natural or synthetic water is applied.

5) Basically, that there is no substantial valid argument for natural versus synthetic regarding "trace"-elements.

A Conclusion:

  This is not the whole story either, but it serves to illustrate the point. In my opinion, unless you're dealing with very large volumes of water, want to devote yourself to adequate preparation and monitoring, and consciously intend to put up with the vagaries of potential pollution, pests and parasites, steer clear of natural water. Synthetic is more convenient, cheaper in total cost, easier to deal with physically, lasts longer, is safer, and it works!

Some folks assert that natural water must be best for "natural" livestock. Maybe they think their aquaria are "little pieces of the ocean"; most systems more closely approximate "little sewers". There is nothing phony about using a synthetic salt mix in an artificial environment.

A few references and further readings are offered listed to elucidate the history of this "controversy", other arguments, and to provide a general background on seawater itself:

Anon. 1974. Seawater. Aquarium Digest International. 2 (4) 1974.

Carlson, J. 1988. Seawater: What Every Marine Aquarist Should Know About This Fluid. Marine Fish Monthly. 3 (4) 1988.

Dawes, J. 1989. The Salt Connection. Seascope. Volume 7, Spring

Fenner, B. 1989. Frequent Partial Water Changes. FAMA April 1989.

Moe, Martin A. 1999. The chlorine trick. FAMA 4/99.

Mowka, E. 1979. Essential Chemical Elements. FAMA. April, 1979.

Mowka, E. 1980. A Fresh Look At Salt Water. FAMA. June, 1980.

Segedi, R. & Kelley, W. 1971. A New Formula for Artificial Sea Water. Marine Aquarist. 2 (8). 1971.

Spencer, G. 1974. Advantages of Unnatural Water! Marine Aquarist Magazine 5 (1). 1974.

Scripps Institute of Oceanography, La Jolla... Free, natural, sand-filtered seawater   7/6/10
Is there any reason not to use the filtered sea water that the Institute provides to the public, for a salt water aquarium at home?
<Mmm, yes... unless one is religious re storing, bleaching... otherwise thoroughly treating such "raw" water, there is danger of introducing undesirable life... And pollutants... And such natural water has little buffering capacity, insufficient biomineral content for "reef" hobby use by and large (which can be made up by supplementing, testing...). And it's a royal pain to go get, lug about and move to your system. Please read here
re: http://wetwebmedia.com/seawater.htm
and the linked files above>
<Welcome. Bob Fenner, who... has used many gallons of this water, and many more gallons of synthetic>

Re: Scripps Institute of Oceanography, La Jolla, free SW   7/6/10
Thanks Bob. That's very helpful. I have a lot of freshwater experience but am thinking of setting up a saltwater tank.
<An adventure for sure! Even more "art" (or voodoo to some) than fresh!>
I just bought your book The Conscientious Marine Aquarist and look forward to reading it.
<Ahh, am sure you will enjoy, gain by its perusal. Cheers, BobF>

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