Of all the many things involved in buying a new fish for the tank, one of the most important is allowing your new fish a chance to get use to the new enviroment. 

The reason for acclimating ant livestock is to allow it to get use to temperature, pH, salinity. When you bring your next fish or coral home, test the water in the bag the fish arrives in, then check the water in your tank. Mostlikely you will find a difference in temperature, pH, and salinity between the two. 

Why should you acclimate for these differences ? The affects of changes in temperature, pH, and salinity on a fishes metabolism, bodily functions, mental and psychological state can be stressful enough to allow parasites to take advantage of a fish, or /and ultimately, cause its death. 

The greater the variance between the two waters, the longer the acclimation period should be. It is suggested that a minimum of 30 minutes, up to 60 minutes is the proper amount of time. During this time water from the tank should be introduced into the bag. This can be done in many ways, but gradually is the key. 

One last note, dont introduce the bag water into the tank ! Who knows whats in the bag water. Certainly nothing other than the fish should come out of the bag.