When transferring saltwater inhabitants from one aquarium to another it is never as simple as floating a bag for 30 minutes and plopping them the tank. Each saltwater system can be completely different. Each could have differing nutrients levels such as phosphate and nitrate, or have higher or lower alkalinity and calcium. Most importantly, they may have a completely different salinity (salt concentration).
In fact, often saltwater fish are kept in ‘hyposalinity’ holding tanks or display tanks at your local stockist. Hyposalinity refers for a lower salt concentrations. A reef tank is normally maintained at a specific gravity of 1.023 – 1.025, but stockists display systems are regularly maintained at a specific gravity of 1.008 – 1.009. The reason they are kept at a lower salinity is that many fish parasites, including marine white spot, cannot tolerate it these levels. It is simply a method of parasite control.
It is worth knowing that invertebrates, corals, live rock and most other saltwater critters will die in hyposalinity. These will always be kept at about 1.025 at your LFS.
Acclimation is simply replacing the water they are accustomed to (in the fish bag) with water from your tank. We need to acclimate saltwater fish and invertebrates by introducing them to their new home in a controlled manner as described below.