Fish are often anesthetized for just a few minutes for diagnostic procedures like skin scrapings and gill biopsies. In these cases, veterinarians have traditionally used what is known as an inexact addition of anesthetics "to effect." In this approach, the anesthetic is sprinkled or poured into the water until the fish loses balance and becomes non-responsive. The needed procedure is then performed as quickly as possible and the fish is returned to anesthetic-free water to recover.
For more information, please read Fish and Anesthesia