I'll also have to agree with the others in that you must make sure your tank is properly cycled (usually I let it run through for 2 weeks to a month depending on the setup).
As for adding new fish for freshwater. Usually letting them float for 30-45 min in there bag is good enough. But to really ensure their health, I would still open the bag, clip it to the tank, and slowly add the tank water to it during the 30-45 min time span to ensure that not only temperature, but water parameters are acclimated for the fish as well.
To make sure it's seen. And to save me typing.
I do the adding more water at least twice, about every 20-25 minutes. About a third new water each time.