When I was in college, as a demonstration, my ichthyology professor used a compressed N2 tank to show a fish reacting to low dissolved O2 levels. He bubbled in the N2 and it displaced the O2 from solution. So outside of it not helping your plants it would be bad for your fish as well.
Note: This is not something that happens with injected CO2 as it follows a different disolution pathway than N2 and O2 so please don't panic and turn off your CO2 injection systems. I.E. Read this post as why not to inject N2 and don't try to infer any of it to injecting CO2.