Apparently it used to be filled with tap water.
That changed in a 2009 reno to use water from the Tidal Basin(man made resevoir) constantly circulating to avoid stagnation issues.

However, that itself caused issues. Rampant algae growth immediately became an issue, so they had to improve the ozone disinfectant system in the same year.

It was drained in 2017 to solve a parasitic snail outbreak which was killing ducks in the pool.