Or if you want to be even more cautious (if you have sensitive fish) you can drip acclimate them. You do this by first placing the bag with the fish in the original water in something to hold it (I use a pitcher). Next, tie a few knots in an airline tube and simply create a siphon with it, siphoning the new water out of the tank and into the bag with the fish in its old water. You will have to play with the number of knots and how tight they are in order to get it to drip just right. This method is recommended for more sensitive fish, and I believe is the way most all saltwater fish keepers acclimate, although I think it is a less stressful way to acclimate any type of fish.
Once you think the water in the fish's bag is mostly your water you can bet the fish and release it into the "new" tank.
