The most common method for introducing new fish into an aquarium is to float the plastic bag for a while, then gradually mix the aquarium water with the fish store water in the bag. It works for many species, but there is a better way of introducing more sensitive fish into a new aquarium. The drip acclimation method sounds complicated; it's really not. You can even buy a cheap drip acclimation kit from the on-line stores. Or you can make one.
Get about four feet of aquarium air hose, and put a regular air valve on one end. When you get the fish home, put them along with the aquarium store water into a clean plastic bucket that holds a couple gallons. Make sure to cover the bucket so the fish can't jump out. Open the air valve all the way, and fill the air hose with water. Clip it to the inside of the aquarium that will hold the fish, and let the water syphon into the bucket. Adjust the output to a drip so the bucket takes about an hour to fill up most of the way. When the bucket is about full, remove the syphon and gently dump the fish into the aquarium.
If you buy one of the drip acclimation kits, they have a rigid, u-shaped tube on the intake end, so you you don't have to bother with clipping the intake to the aquarium. They also have a plastic valve on the output end.
Drip acclimation will reduce much of the stress associated with moving fish from one environment to another. In my experience, it's especially good for angelfish.
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