Natural seawater or NSW is as the name suggests water collected from an ocean or bay or other natural source of clean seawater, (see Collecting Natural Seawater for more information on where to collect NSW.
Why Use Natural Seawater
There can be many benifits to using NSW
Buying NSW from your LFS can often be cheaper than buying the salt mix to make ASW (see Mixing Artificial Seawater ) as the salt mix may cost from 20c to $1 per litre and then theres the fresh water needed to mix it, where as NSW from a LFS may be as cheap as 10c to 30c per litre so if you have the means to transport it can be a very cost effiecent.
You can also get NSW delivered in some places ranging from 8c to 20C per liter yet minimum quanities may apply and you will need a place to store it, however storing water for your tank is a good idea expecaily if you have a large system for regualar and emergency water changes.
You can even collect your own if your lucky enough to live close to the coast that could drop the costs including petrol down to 4c a litre
Chemistry of Natural Seawater
What we all hope to do in our aquriuams is emulate the environment of the ocean where our livestock orignaly lived, so the make up of the water is important, the disolved components in NSW such as sodium, calcium, magnesium, boron and many others are all correct where as ASW is just an approximation of the chemicals and may not be exact. http://saltaquarium.about.com/cs/seasaltmixes/l/aa090503b.htm shows information on the makeup of ASW vs NSW
If you get NSW delivered and have it on hand ready to use when you need it then its there already made ready to go into your tank, without having to mix up salt mixes and wait overnight for them to settle, even if you have collected your own nsw and have it stored its still ready for use in the case of an emergency when you need the water ready to go right away.
If you live close to the coast then collecting natural seawater can be a cheap and relatively easy way to provide water for water changes. The site chosen to collect from has to be clean and close to the ocean, not near river entrances and other potential sources for contamination. Never collect after heavy rain, as this will have chemicals and pollutants on the land wash off into the water and there is a risk collecting these potentially harmful substances.
You will need equipment to collect the water and a place to store the water when you have it at home. See Collecting Natural Seawater for more details on where to collect and the procedures and methods to use.
Storing Natural Seawater
Natural seawater can be stored indefinitely, if the right methods are used.
The simplest way is to store it in a clean plastic container, such as a rainwater tank (avoid brass fittings), IBC tanks (1,100 lt) or drums for water storage / olives etc. It needs to be sealed so that things cannot get in (dust, leaves, rats, insects etc) or out (evaporation). It does not have to be stored in darkness, but typically the plastic will last a lot longer if it does. Many of the plastics will degrade within a couple of years if exposed to the full sun.