Acid Pool is the first upgrade on the bottom path for the Alchemist in BTD6. It allows the Alchemist to create small, short-lived pools of corrosive acid near it to damage bloons when none are within its radius or if the Alchemist misses. Each Acid Pool can soak up to 5 bloons or last for 7 seconds before disappearing, whichever comes first. It costs $510 on Easy, $600 on Medium, $650 on Hard and $720 on Impoppable.
Generally Acid Pool can seem a bit weak, but the acid pools can be crosspathed with middle path to improve damage. While still fairly average, it is a cheap way to slightly improve your MOAB and Ceramic popping power.