I filled the vents (in the Toxic Gas rooms) with water, and it really worked.

That means Aqua Brews have another use: get free Potions of Purity! I usually save Potions of Purity to deal with the Rotting Fist + another fist combo (especially ones with ranged attack ability). When you enable the Badder Bosses challenge, two fists will appear at the same time).

(I only have the screenshot of the result. Sorry).