Acclimatisation
The process of gradual adaptation of fish and invertebrates to the water parameters of the new aquarium. Skipping acclimatization can cause fatal osmotic shock.
Detailed explanation
When you buy a new fish the water in the bag has different parameters from those in your aquarium: pH, temperature, salinity, hardness can differ quite a lot. The abrupt transition from one water to another causes osmotic shock which can kill the fish in a few hours.
The simplest method is floating: leave the closed bag floating in the aquarium for 15-20 minutes to balance the temperature. Then you open it and add a little water from the aquarium every 5 minutes for 30-40 minutes. Finally, transfer the fish with a net without pouring the water from the bag into the aquarium.
The drip acclimation method is more precise. Empty the bag into a bucket and drip the aquarium water into the bucket with a tube set at 2-3 drops per second. Continue until the original volume is tripled (approximately 1-2 hours). This method is mandatory for marine invertebrates (shrimps, crabs, urchins, starfish) which are very sensitive to differences in salinity.
Corals usually only acclimate to temperature and light. They position themselves low in the aquarium and gradually rise over a period of weeks.
Newly acclimated fish should be left alone. No bright lights, no food for the first few hours. If possible, turn off the lights in the aquarium on the evening of the introduction.
Never acclimate a visibly ill fish. If you notice white dots, eroded fins or abnormal behaviour in the bag, it is best to put it directly in quarantine.