Besides, the reflecting pool contains 6.75 million gallons of water. Are they really stupid enough to believe a few bottles of hydrogen peroxide will have any effect on the algae? Apparently, they are.

us taxpayers? One 50 pound bag (approximately $140.00) would treat that for over a month.

As the least costly aquatic herbicide, copper sulfate pentahydrate is widely used for short-term management of algae. It is ineffective in the long term unless combined with other management methods. Both filamentous and planktonic algae are simple plants with the capacity to rapidly reproduce and become overabundant. They can grow back quickly following herbicide applications.

Copper is seldom effective against weeds other than algae. Be certain of the identity of any aquatic weed before selecting a herbicide.

Many local farm stores and online supply sources keep copper sulfate crystals (aka “bluestone”) on hand. Copper-based liquid aquatic herbicides offer some advantages but are a more expensive alternative.

If copper sulfate or other herbicides are the only management tool used on algae, it typically grows back rapidly. When algal cells die, they break open releasing phosphorous into the water column. Light penetration into the water column increases. Surviving algal cells find this ideal for their reproduction and a new algal bloom quickly returns.
https://extension.okstate.edu/fact-sheets/copper-sulfate-an-often-overused-chemical-in-pond-management

I also add blue water dye to suppress new algae growth. The dye shades the water limiting the sunlight (specifically red-orange and blue-violet spectrums) that underwater weeds and algae need for photosynthesis.

It's also toxic to wildlife like ducks and geese, and can leech into groundwater (the pool still leaks like a sieve). They probably would have used the stuff years ago if they thought it would work.

Find the right amount that safely treats that much water, and you have the cure. It's safe to swim in a pond right after a treatment.