Saltwater is a major component of the reef tank. Virtually all reef tanks use a dry salt mix to create the saltwater used in them. This is not concentrated salt from the ocean, but instead is a carefully formulated salt developed from scratch. For those who live in the tropics (I used to live in Hawaii), you can collect fresh saltwater, but you have to be careful to do this away from sources of pollution. Saltwater from cold water areas is generally unsuitable since they often contain a high zooplankton component that will rapidly deteriorate in the tropical temperatures of the reef tank. There is also the chance that salinity will vary widely if there is freshwater runoff near the collection site.

There are a number of brands of salt mix on the market to chose from. I have used several different ones including Tropic Marin, Instant Ocean, Corallife, Oceanic and Kent.   Probably one of the best is also one of the cheapest (how often does that happen?) and that is Instant Ocean. It is easy to pick one of the higher priced, fancier looking brands, but it is not necessary. Analysis of the salt mixes that have been performed by others have shown that Instant Ocean (IO) as it is often referred to has as optimal a formulation as any of the other salts and is better than most. IO has a sister brand called Reef Crystals that is more expensive and has elevated levels of calcium when mixed. The added cost of this product does not make much sense since these levels can easily be adjusted using cheaper methods.  Kent seems to be popular and is what I currently am using in my own tanks.  Oceanic salt has had reports of incorrectly mixed batches causing tank crashes when the salt was first introduced, but seems to be stable now.  It does tend to mix up with a lower alkalinity than is ideal for SPS tanks, but is relatively inexpensive.

Freshly mixed saltwater should not be added to a tank unless it is a relatively small amount.  Larger amounts (perhaps more than about 5% of the tank volume) should be mixed for about 24 hours or so before adding to the tank.  The reason is that freshly mixed saltwater seems to be somewhat caustic for reasons that no one really understands.  In my own case, I use a tub to mix my saltwater.  I place a heater and a powerhead in the tub and let them run overnight when I make new saltwater.

When mixing the saltwater, it is very important to use purified water. Most public and well water sources contain excessive amounts of chemicals, elements and compounds that can cause problems in the reef tank. Common undesirable additives that frequently come free with water include phosphates, nitrates and silicates, all of which can cause excessive algae growth. This becomes even more important for replacement water (water which must be replaced due to evaporation). Since the chemicals do not evaporate, they tend to get added with each water addition and accumulate in the system over time. So even if the basic water concentration of impurities is OK, they can still accumulate to levels that are not OK. Some people have found that they can use tap water as-is without problems, but you should not do this unless you are willing to take a significant risk. I have learned this lesson the hard way.

There are a number of ways to get purified water. One way is to go down to the local grocery and buy water sold as distilled or purified. This works fine, especially for small tanks where water consumption is minimal. You generally pay $.50 to $1.00 per gallon for water in this form. Another way to obtain purified water is to buy it from your LFS, if they provide it as a service. Most LFS have purification equipment to supply their own needs and are happy to sell the excess, usually for around $.25/gal. Most people, however, eventually bite the bullet and buy their own water purification system. The ones typically sold at home improvement centers are not very well suited for our application as they provide incomplete filtration. It is better to purchase a system designed for the purpose of purifying water for reef tanks. These systems are called RO (Reverse Osmosis) or RO/DI (Reverse Osmosis / DeIonization) filters. The addition of the DI unit provides enhanced filtration and is usually required to remove silicates that can cause unwanted diatom growth. These systems typically produce between about 25 and 100 gallons a day of purified water. The cost of the system is dependent on the gals/day rating of the filter and the number of filtration stages it has. A 25 gallon a day unit produces about 1 gallon of purified water an hour. It also produces perhaps 10 gallons of waste water in that same hour. This waste water can be used for other purposes and will be slightly harder than the normal tap water.

Further Reading:

Understanding Seawater 
By Randy-Holmes Farley

 The Composition Of Several Synthetic Seawater Mixes
By M. Atkinson & C. Bingman