Question

only 2 percent of the oxygen intake in humans is used by respiratory muscles. do you think the percent of oxygen used by respiratory muscles in a fish is higher or lower than this

Answer 1

Air has a small mass & viscosity and contains 20% oxygen, while water has a much higher mass & viscosity and contains less oxygen (~5%).
What do these considerations alone tell you?

If you wish, further reading & discussion can be found here:
http://www.lookd.com/fish/respiration.html