Question

Why doesn't fish cells swell out when exposed to a hypotonic solution?

Answer 1

A hypotonic solution is any solution that has a lower osmotic pressure than another solution. In the biological fields, this generally refers to a solution that has less solute and more water than another solution.
the fish lived in sea is swell when we put it into a normal water becaure compared to sea fish its out side is hypotonic ,

for fresh water fish it doesnt swell because it is adapted for the condition of hypotonic solution