Question

the fish weigh less in water than in air ​

Answer 1

Fish weighs less in water as compared to air because of the buoyant force. As the fish swims , some of the water is displaced by it and thus the water offers an upward force ..

Answer 2

Fish  weighs less in water as compared to air because of the buoyant force.

As the fish swims , some of the water is displaced by it and thus the water offers an upward force

Not only fish all things weight less in water.This is because the density of water is more than that of air.And the bouyancy of the object makes it less weight by pushing upwards 

hope it helps