To actually explain it, red wavelengths are shit at going thru water
Send light to the bottom of a pool and it reflects back as just blue off white.
Send it to the bottom of even a shallow pool that’s dark blue, and you get back black. Which warms the water and helps algae. With the green algae, you get even blacker water.
It has nothing to do with how pools work…
It’s about how visible wavelengths work…
Edit:
To actually explain it, red wavelengths are shit at going thru water
Send light to the bottom of a pool and it reflects back as just blue off white.
Send it to the bottom of even a shallow pool that’s dark blue, and you get back black. Which warms the water and helps algae. With the green algae, you get even blacker water.