Sounds simple enough, but there is more to it of course. The very first step in making up a new batch of fresh salt water is to make sure your source of fresh water is clean. Depending on where you live, the city's tap water may not be adequate for use in reef aquariums. The water may contain unusually high levels of nitrate, phosphate, or worse yet, heavy metals. This is not to say all tap water sources are bad. Many aquarists have been successful using straight tap water, but it is more location dependent than anything. If you are worried or know that the city water is unacceptable, it is a good idea to invest in a tap water conditioner designed for the hobby.

There are two types of tap water purifiers:
1. Reverse Osmosis Filters (RO)
2. De-ionization Filters (DI)

A Reverse Osmosis filter employs one of two types of membranes, cellulose triacetate (CTA) or thin film composite (TFC). TFC membranes do a better job of filtering and are the recommended choice for reef aquariums. RO filters work by forcing water through the membrane under pressure. The membrane traps particles and allows the clean water through.

There are two disadvantages to RO filters:
1. They waste a lot of water. For every "clean" gallon of water, three or more gallons are expelled as "waste" water. The "waste" water is sometimes used to water plants or in the laundry. After all, it isn't any dirtier than what comes out of the faucet, right?
2. The filters need to be replaced every 6 months or so depending on how much the membranes were used. To really gauge how effective the membrane is, a Total Dissolved Solids Meter (TDS) comes in handy. With it you can see when the filter's "clean" line has the same total dissolved solids as the "waste" line.

De-ionization filters use anion and cation resins to filter out negatively and positively charged particles respectively. The resins have to be recharged or replaced as time goes on since they lose their potency as they bind more and more particles. They do not suffer from the disadvantage of "waste" water, however they expire more quickly than a RO filter.

Hybrid units are available that use a reverse osmosis pre-filter that then sends the clean water through a de-ionization filter.

Once the source of fresh water is adequate, we can finally mix the salt in to begin making salt water. What is normally done is to make the new water in a five-gallon bucket. The salt must be completely dissolved before it can be dumped back into the tank. For the first couple of hours, freshly made salt water is stressful to fish and coral. It should remain in the bucket for a minimum of a few hours with constant agitation. Water movement can be easily achieved by placing a small water pump in the bucket. Some reef keepers leave the new saltwater mixing overnight and do the actual water change the next day. To avoid the difference in temperature, they install a heater in the bucket matched to the temperature of the tank.