Don't use hydrogen peroxide for pool algae.

I wouldn’t. Hydrogen peroxide can kill algae in a pinch, but it’s not the right tool for the job. It breaks down fast in sunlight, you need a pretty high concentration to be effective (which can be dangerous to handle), and it’s actually more expensive than standard pool algaecide. Plus, if you have any chlorine in the water, they react with each other and waste both chemicals.

A good algaecide or a proper chlorine shock treatment will do a better job with less fuss. Hydrogen peroxide makes more sense for a small fountain or a birdbath than a full swimming pool.

Stick with the stuff designed for pools.