I use it myself but mostly to keep algae away. I put about a table spoon or two into a gallon of rain water and shake it up. I only use this on my inside plants though, outside I don't care if the trays get algae.
Thanks for the great info.
Edit: to answer your question, I think adding it to the rain water will help with algae. H2O2 is commonly used with planted fresh water aquariums both as a plant dip and spot treatment in the tank to kill algae.
When you treat for algae do you place it in a spray bottle and spray the algae directly?
What's a plant dip?
For my in door plants I don't treat the algae, I more keep it from occurring by adding a little peroxide to the jug of rain water to kill off any algae. I haven't got any algae on top of any of my pots, only a little white mold that went away on it's own. But then again I do use that same jug of water to refill my spray bottle so maybe that has some effect.
When you buy fresh water plants there is always a chance that they have algae, even if you can't see it. If you just put them in your aquarium now you've introduced the algae and it can run wild, especially if you're running a lot of light and high nutrients. A plant dip is when you mix some peroxide and water and dip your plant in it for a few minutes to kill off any algae that may be on them before putting them in your tank.