what is the benefit of having high oxygen levels in a fish tank?

[torresraulg]

I'm not sure about "high" oxygen levels, but fish need oxygen to live.
I would not put hydrogen peroxide in a fish tank as it is primarily used to kill bacteria. In an aquariums case, beneficial bacteria. Even in low quantities I would not risk putting it in. Try adding air stones instead.

 

[VIVEK]

will adding hydrogen peroxide in very very low quantities help

 

[jtrusnik]

Hydrogen peroxide won't do much, and it could kick off a pH imbalance to contend with. A much safer and easier method would be to disturb the water surface, such as with a filter outflow or a powerhead. This will increase gaseous exchange, and thus increase the oxygen in the water.