Freshwater fish generally have a higher concentration of salt in their cells than in the surrounding water. Due to this difference in concentration, water flows through the membrane and into the cells permanently (in the direction of the higher salt concentration), as a result of which this absorbed water must be excreted actively again into the surrounding water by the cells as individual components and by the fish as a whole, since they would burst otherwise. This balance between the influx of water and the permanent discharge of water is subject to fine control and maintained with a high expenditure of energy.If fish are moved from water with a higher salt concentration/conductivity/hardness (not to be confused with salt water) into water with a lower salt concentration/conductivity/hardness too quickly, the osmotic situation will change suddenly and significantly more water will penetrate into the fish and its cells. Detachments of the mucous membrane, bursting of blood cells and other cells all the way to injuries to the cartilage tissue may occur. If these injuries do not cause the fish to die quickly, secondary infections from bacteria and fungi, which will afflict the fish, must be expected.

Remedial action: Only transfer the fish to a different body of water after a period of slow adjustment. The total salt content of the transport water and the tank water can be determined with the aid of a conductivity metre. The difference between the two bodies of water should not exceed 300 µS/cm. If the difference is greater (the tank water has a lower hardness/conductivity value than the transport water), the water should first be treated with specific water conditioners if you have cichlids from the East African lakes in order to balance the water parameters. After the fish have been transferred, the salt content/electric conductivity/hardness can then be adjusted again by approx. 300 µS/cm per day by using osmosis water.The reverse process, i.e. a rapid increase in the salt content in the water, is tolerated with no problems by fish.