When you select your fish at your local pet fish store, you need to go directly home to get your fish to their new home. This is because you do not want to have your fish overheat if the weather is hot or become cold if you are buying your fish in the dead of winter.
Most fish stores will put your new fish in a plastic bag, a puff of oxygen and secure it with a rubber band.
What to Prevent
When you add your fish you want to accomplish two goals:
Adding New Fish To A New Tank
How To Prevent Shocking Your Fish
Any sudden changes in water temperature, even by as little as one degree, can shock your fish leading to disease. So to prevent this, float the plastic bag that has your fish in it in you aquarium for up to 20 minutes.
This will allow the temperature in the plastic bag to equalize with the temperature in your aquarium.
Prevent Contaminating Your Tank
Freshwater aquariums have their own eco-system. When you set up your aquarium and even after your aquarium is running, you need to wash everything you put in it with warm water to prevent contaminating you aquarium.
Preventing contamination of your aquarium includes stopping cross-contamination between your aquarium and other aquariums. So you do not just want to empty your new fish and the pet store aquarium water in your aquarium.
To prevent this, once the temperatures are equalized, use a small fish net to remove your fish from the plastic bag and gently place your fish in your aquarium.
Adding Fish To An Established Tank
Once you have your aquarium up and running, you should have a quarantine or medical tank. This tank should have the same water and be at the same temperature that is in your display aquarium.
Do the same with your new fish that you did with the display tank, equalize temperature by floating the plastic bag your new fish is in and then using a small net put your new fish in the quarantine tank.
Locate the quarantine tank next to your display tank to allow your new fish and your established fish get use to each other. After about a week you can place your new fish in your display tank.