National Park Service workers late Thursday were still cleaning up dead blue gill, bass and carp from the Constitution Gardens pond on the Mall, where thousands of deceased fish were discovered floating in the water Wednesday night.

Workers in waders were scooping up the fish in nets and placing them in plastic bags. The water in the concrete-lined pond, just west of the World War II Memorial, was a dark green, and the dead fish gave off a rank aroma.

It’s not known what killed the fish, said Park Service spokeswoman Carol Bradley Johnson: “We’re testing the water now.”

“This happens from time to time, because it is a closed system, and it’s shallow and it gets hot,” she said.

“You get algae, and you don’t have enough oxygen.”

Fish are seen in the Constitution Gardens pond on the Mall. (Michael E. Ruane/The Washington Post)

“It’s hard to maintain an ecological balance,” she said. The pond was “designed in the ’70s, and it was a design flaw.”

The Park Service wants eventually to reconstruct the pond to make it self-sustaining.