Fresh water is a hypotonic solution for saltwater fish. Pure water is a hypotonic solution for plant cells.
A hypotonic solution occurs when the concentration of the solute outside of a cell is less than the concentration of solute inside of the cell.
Remember that hypo means under. So hypotonic solutions have a concentration of solute (in this case salt) which is lower than the concentration of salt inside of the cells of the fish. Keep reading to learn more…
One of the reasons that saltwater fish don’t like freshwater much is that they are used to living in seawater with its high salt content (salt = solute). When they are placed in fresh water their cells (particularly those in their gills) have much higher salt concentrations than the water which causes water to rush into them via . Unfortunately for the fish this can cause the cells to burst. In this example fresh water is a hypotonic solution for saltwater fish.
Here is a video which shows what happens to onion cells in both hypotonic (pure water) and hypertonic (salt) solutions.

Hope this helps!

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