I stretched a fine 1 mm mesh over my water butt inlet just to keep the leaves out: since then, not a single larva in the water and it stays clear all summer
A simple square of 1mm mesh stretched over your water butt inlet does far more than keep leaves out—it stops mosquitoes from ever breeding in the first place. By denying female mosquitoes access to the water surface, you eliminate the entire breeding cycle before it starts, and your water stays clear all summer.