Flocculant (floc)

Flocculant, or 'floc,' binds the microscopic particles that make water look hazy into larger clusters heavy enough to settle on the pool floor instead of passing through the filter. Once they've settled, usually overnight, you vacuum them out to waste rather than through the normal filter cycle, since most filters can't catch particles that fine. It's the fastest way to clear genuinely cloudy water, but it's a manual, one-time cleanup step, not a routine maintenance product, and the pool typically can't be swum in during the settling period.