Try to get your new fish home from the shop to your tank as smoothly, and with as little disturbance, as you can.  They are pretty resilient, and will be in their little cocoon of water and plastic, but they won’t be overjoyed to be bumped about on the way home, so try to keep unnecessary jolting to a minimum.

Bear in mind that any stress they have experienced on the way home will have led to an increase in ammonia production, so don’t just dump them in your tank in the water they came in.

To get the temperature to the correct level of your tank, float their bag, unopened, in the tank for twenty minutes or so, then you can start introducing them to their new home.  But don’t rush!

Open the bag carefully with scissors, and attach it to the side of the tank (washing pegs come in handy here.) It isn’t just the temperature that may be different in your tank to the conditions they have been living in.  Although if your home is fairly close to where you bought your fish, the water make-up should be similar, there may be slight variations, so it’s best to acclimatise them further by gradually replacing the water in their bag with your own tank water.

With the bag open and secured, carefully remove a small amount of water from the bag and discard it completely (not in the tank), replacing it with tank water.  Do this gradually, in several goes, using either a small glass or a turkey baster (taking care not to scoop out the fish!) over half an hour or so, then, once the bag is pretty much all tank water, carefully dip the bag into the tank, or net the fish out, letting them swim into their new environment.

It’s generally recommended to introduce new fish into an existing set up with the lights off for the duration of the process.  They can be switched on again once they have found their way around (after a few hours).