I realized today at work how many times a day I tell people this and thought maybe I should post it here. Let me start off by saying, I think its very important to quarantine all new fish that you get before introducing them into your main tank. I know not everyone does though and thats where I thought this would come in handy. (I dont quarantine all my tropical fish, just goldfish).
Sure, everyone knows about floating the bag in the water for half an hour or so, but do you realize how important it is to adjust your new fish to the pH in your tank? A lot of customers were telling me they were floating the bag but still losing fish, so I started thinking about pH shock.
What I do when I get a new fish is, take one of my fish buckets (fish only, never had soap/cleaners in them) and pour the water and fish gently into that. Then over the next couple of hours (make sure you have aeration if the fish are large) I slowly pour some small cupfuls of water from my tank into the bucket. That way they will adjust to your temperature, pH and water quality. After awhile of doing this, I then scoop the fish into a net and then place them in my tank. That way you wont worry about getting any pet store water in your main tank.
Hope this helps!