So you've brought your new fish home and now need to acclimate it. How do you acclimate fish? Well, fish are as sensitive as humans to sudden changes in water temperature. Have you ever been in a hot shower and had the water turn cold on you? You probably didn't enjoy it. Your new fish may do worse than not enjoy the change in temperature. Disease, ich or death can occur if you don't take the time to acclimate your fish properly. Go slow. It's very easy to do.
You'll first want to distract any other fish you may have in the aquarium by feeding them. Turn off the light in the fish tank and float your bag containing the new fish in the water for a 1/2 hour. Make sure the bag is still tightly sealed! This will slowly acclimate the water in the bag to the water in your fish tank.
The next step is introducing the aquarium water. I've found the best way to do this is by placing the bag in your fish bucket with some of the aquarium water. You can leave it in your fish tank, but watch for jumping fish (I had a yoyo loach leap from the bag on to the floor once!), or a bag that rolls. You don't want to add the pet store water to your aquarium. (Please note: I use the fishless cycling technique when initially adding fish. A reader saw that I had a lot of bags in the picture, and thought I was adding too many at once.)
Add 1/3 of the aquarium water to your bag water. By using the bucket you just dip it in a little. Wait 10 minutes, then add a 1/3 more. After 10 more minutes your new fish should be acclimated. The slower you do this process the better. Take about 45 minutes to an hour when you acclimate saltwater fish. Net the fish and place him in your aquarium.
You've avoided mixing the fish store water with your tank water and the fish is acclimated to his new home.