despite the fact that 70% of the atmosphere is nitrogen, are bodies dont have a use for it, our bodies use the oxygen (for the krebs cycle i think) and blow everything else out, plus CO2
if anything, nitrogen prevents us from breathing in too much oxygen
as far as the fish part goes, probably the same thing it would do to us, fish (from what i understand) breath in H2O, and tear the hydrogen from the oxygen, and (again from what i understand) and breath in almost 100% oxygen (after the hydrogen is torn off). opposed to us, out of all the air we breath in, only 20% is nitrogen.
fish are more sensitive to changes than us, as they are use to breathing in pure oxygen, (and probably some CO2
, but not alot), and interupting that would mess them up
again, this is speculation