Here's why:
* Seawater is salty: The ocean is full of dissolved salts, making it hypertonic (higher salt concentration) compared to the fish's internal fluids.
* Freshwater is less salty: Tap water, on the other hand, is hypotonic (lower salt concentration).
* Osmosis: When a fish is placed in freshwater, the water rushes into its body through osmosis. Osmosis is the natural movement of water across a semi-permeable membrane (like a fish's skin) from an area of high concentration to an area of low concentration.
* Swelling: This influx of water causes the fish's cells to swell, potentially damaging tissues and organs. In extreme cases, the swelling can be so severe that it leads to organ failure and death.
It's crucial to remember that:
* Sudden changes in salinity are extremely stressful for marine fish. Even a small amount of freshwater can cause harm.
* Even if a fish survives the initial exposure, it may suffer long-term health problems.
If you're looking to keep marine fish, you need to:
* Use a saltwater aquarium: Maintain the appropriate salinity level (which varies depending on the fish species).
* Never use tap water directly in your aquarium.
* Consult a veterinarian or aquarium specialist for advice.
Let me know if you have any other questions.