It is vital that you add new fish slowly to your aquarium. The key is to move slowly so that new fish have a good chance to properly acclimatise to the new surroundings. One of the most common mistakes which can be made is simply pouring new fish into a tank all together. This is not a great plan, as the fish can easily become stressed by sudden changes in water temperature. The same goes for new shrimp and snails too.
Fish are cold blooded animals, which means their body temperature is directly related to the temperature of the water they live in. It is a good idea to float the bag the fish come in in the aquarium water for around 20 minutes or so to equalize the temperature with that of the aquarium. Once the bag feels a similar temperature to that of the aquarium, yuou can release the fish.