How to act urgently to deflate a freshwater fish suffering from an osmotic disorder?

Following an alteration of the skin or branchial epithelium by compounds such as ammonia or by ectoparasites, freshwater fish can begin to swell (cutaneous edema, dropsy), by osmotic imbalance between the internal environment of the fish and the external environment.

To “deflate” the fish, it can be placed in water with a salt concentration (essentially chloride of sodium) is artificially increased above 3 grams per liter of water. Classically for carp and goldfish, the salinity is increased to between 5 and 7 g per liter (i.e. 5 to 7 kg of salt per cubic meter of water) gradually over several days.

The increase in salinity can be rapid (2 grams per liter per day). On the other hand, the reduction in salinity once the fish has deflated should be slow (2 grams per liter per week).