Hydrogen peroxide in an aquarium is like alcohol in a human. A little just causes a few cells to die, too much and everything can die.
I used some in my frag tank to help get rid of some nuisance algae. My first try was 1ml per gallon and I shut off the flow in and out of the tank for a hour. It had a small affect. 2 weeks later I tried 3ml per gallon and it cleared out the algae. All my corals were OK, even some high end acro and digi frags. But I had a 1 1/2" pvc end cap full of pulsing xenia and it was reduced to very small blobs of goo in the bottom of the cap. It took a good 2 weeks, but the xenia is coming back slowly.
Moral of the story, be careful and start small!