When you add new fish to your tank, you may be tempted to just slip them in and hope for the best. This is generally not the best way to add fish to an existing aquarium! There are some things you can do to make the transition easier for the fish.
The first thing to think about is the temperature of the water. The fish will come in bags with water in from the shop. These are likely to be a completely different temperature to the water in your aquarium. As fish are cold blooded, it is important that you match the temperature before releasing the fish. You can easily do this by floating the bag with fish inside in the water of your tank. After 20-30 minutes, you can see if the water matches that of the tank.