This is a video showing what happened when I added Hydrogen peroxide to my pond.
I used 3% hydrogen peroxide heres a link- https://amzn.to/3RGlfI4 (Amazon affiliate link).
You can also buy it in 6, 9 and 12, 20 and 50% but that will change the dosage requirements.
A common guide on the internet states to use 1/2 Cup (125ml) of 3% hydrogen peroxide per 100 gallons (378Litres) of water. This is a good starting point from there you can monitor how it interacts with your pond and decide if you want to increase the dosage or use a stronger strength to make the treatment more economical.