Originally Posted by Myates
QTing new fish is always the best choice. It's not one that we necessarily like because, who wants to wait 2+ weeks to put their new pretty fish in the tank? But it is the more ideal way to go.
Also, you don't want to just "float the bag for 20 minutes" either, as that is only half the step of acclimating them- floating will get the temperatures even, but you have to also adjust them to the new chemistry of your tank. Without doing so, you are putting your fish at a high risk of going into os shock because I guarantee you that your water and the tank water at the store won't have the same chemistry.
To safely acclimate them to the new chemistry you will want to remove part of the water in the bag/cup and replace it with water from the tank making sure that the bagged water does not enter into your tank. You can do this by using a small cup to dip out the water from the bag, or gently pour some of the water from the bag into a cup. And just use another cup to scoop water into the bag.
You will have to do that process a few times during the 20-30 minutes they are floating.
A lot of people don't do this, but then a lot of times they end up with a fish that goes into shock and dies within hours of introduction and then wonders why the fish died.
I do basically the same thing for acclimating my fish, I use a tablespoon though. I float the container (bag or plastic cup) for about 15 minutes, then I add a tablespoon of tank water, then I add a tablespoon of water about every 15 minutes for an hour or more. Then I let them free into their new home.