For what it's worth my old page on HP and algae.

Assume you are using food grade hydrogen peroxide, which is very high concentration. The brochure with mine suggests initial treatment of 8 oz per 500 gallons, then maintenance dose of 2-4 oz weekly if in direct sun. I use it in my fountain with great success. They say it can be used on plants.

Where do you get food grade hydrogen peroxide?

Amazon, Ebay, and about a billion other places online. Locally pretty much any store that has the word nature, natural, health, healthy in their name.

Cheaper to get the same high concentrations in non-food grade. Several products are sold specifically for algae but it's all the same. Food grade just makes sure the product stays pure during manufacture.

A much safer alternative would be grapefruit seed extract. I buy on line under the name citricidal. I use it for all kinds of things, including in the drinking water I put by for emergencies.

why not just keep the water storage tank dark so algae can't grow?