how soon did the algae appear after filling the reflecting pool

The algae appeared about one day after the Reflecting Pool was refilled. CNN reported that algae was already coating parts of the newly opened pool the day after the water went back in, and a worker was seen removing it shortly after.

What the reports say

• One report says the algae was visible a day after refilling.

• Other coverage says it spread shortly after the renovation and refill.

• By mid-June, outlets were describing the pool as turning green from a bloom that appeared within days.

Context

The reporting frames this as a quick algae bloom in the shallow, sunny pool rather than something that took weeks to develop. The Interior Department described it as “residual algae” from the startup process.

TL;DR

It showed up very fast — basically within a day of refilling.