Although a fish can take in oxygen, they can only take in oxygen from water; as evident by water's molecular confguration (H)
When the fish goes on land, their gills collapse, causing them to suffocate. This is due to the fact that their gills are not suitably made for taking in air. A high surface area is also needed for aquatic animals in order to take in oxygen, and when they go to land, these things all of these things collapse together, blocking their way of taking in oxygen.
v₁ = v₂ n₁ n₂
given: n₁ = 0.75 mol v₁ = 7.25 l n₂ = ? v₂ = 20 l
solution: n₂ = v₂ n₁ v₁ n₂ = (20 l) (0.75 mol) 7.25 l n₂ = (2.76) (0.75 mol) n₂ = 2.07 mol of nitrogen gas