Hydrogen peroxide is just water with an extra oxygen atom per molecule. It's unstable and releases the extra oxygen and you get small bubbles. These act to make the leaves buoyant hence they come to the surface. It's an oxidising agent and that concentration will be very strong so I can't imagine the algae surviving, let alone other things that might have been in there. But extra oxygen will dissipate fairly quickly and all that will be left is the water component. There'll be no other chemicals, unless there's some listed in the contents on the bottle that's been mixed in.

tropicbreezent ~ Thanks for your reply and the explanation of just what the heck is going on. Awoke this morning to another layer of floating leaves, but at least now I know why. For a senior, this almost seems like the easiest way to get the bottom gunk to the surface after winter

So, while safe of chemicals, will the water now be too alkaline to use? Already deal with a pH of a bit above 7 in most beds, so don't want to encourage that upswing.

Thanks again. annie

It won't change the pH. I'm not even sure that even with a small pond that the concentration is all that high. I wouldn't worry about it. BTW, we have an experimental member who is reasonably confident at this point that string algae will/would successfully compete with single cell algae and therefore prevent pea soup problems. His (and my) point is that a little string algae is a good thing. I like seeing rocks covered with some green. If it were me, I would use a toilet brush to tangle up the algae and just pull it out every once in a while. The algae will probably help trap and then remove other small crud, so the removal would be doing cleaning/filtering as a bonus.

chas045 - I'm glad to hear about the pH. Would hate dumping beneficial dirty pond water on anything but garden soil. Interesting comments re string algae preventing pea soup ~ another scourge. Only reason we resorted to hydrogen peroxide is that we tried the toilet brush method and expected to 'spaghetti twirl' most of it out, but didn't have much success. Not sure why. It;s def string algae ~ just not sure why it didn't wind around the brush like you would expect.

Ok, guys. Always appreciate the help of GW members. You all are the best !

String algae is very slippery. Perhaps you have a slightly different type from mine. Mine will tangle on a branch but I certainly don't get it all.

In water, it looked like long, green hair. Gross. Thought it would be easy to wrap up and scoop out, but not so. Brush was coated in 'green', but not string algae.

Leaves still floating to the top and water looks horrible. Not sure whether to add more HP or just do a partial water exchange this weekend. I never wipe down the sides or rocks, because I like seeing the algae covered rocks, too. Last summer the pond hit a lovely balance of clear water while algae covered the rocks and sides. Shade from a patio umbrella and a couple of quick spreading aquatic plants helped.

Frogs and toads will be frequenting the pond soon to lay eggs, so on a mission to get things right for them before hand.

I know that there are plenty of people who believe in large water exchanges but I doubt except in emergency, that it does any good, and I can imagine why it could be detrimental. I have never intentionally exchanged water. Last summer, I had a leak that I wasn't finding, and was therefore adding more water than usual; but I often don't add any.

I have often found that I can see my fish, frogs etc. thru the string algae. I also sometimes pull some out with my hands. Perhaps you could try that although it seems less effective than the stick or brush, but who knows. It might be worthwhile to let your water equalibrate with some plants and see if things get better on their own.