The 'correct' hydrogen peroxide at the 'correct' strength is 'safe'.
The wrong type at the wrong dose will kill everything.
Somewhere in between the two allows you to selectively kill lesser organisms (algae, bacteria, etc) without harming higher-level organisms. :wink2:
Used in shrimp tanks (they're on the more delicate side of critters) to control algae and bacteria. Getting the dose right to kill the things you want rid of without killing the things you want to keep is the tricky bit of course! Also depends upon the algae type that you're targeting. Have no idea about use in outdoor ponds, but search youtube for "Marks Shrimp Tanks H2O2" and you might find that video a useful starting point.