Hypochlorous acid (HOCl)
Also called: HOCl, active chlorine.
Hypochlorous acid (HOCl) is the active form of chlorine in pool water, the form that actually kills algae and bacteria. A free chlorine test counts all the available chlorine, but only the HOCl fraction does the work, and cyanuric acid (CYA) controls how much of it is active at once.
Why it matters
When chlorine dissolves in water it becomes hypochlorous acid and hypochlorite ion, which is a far weaker sanitizer. CYA holds most of the chlorine in reserve and releases HOCl a little at a time. Richard Falk pinned down that active HOCl is proportional to the ratio of free chlorine to CYA. Keeping free chlorine at about 7.5% of CYA holds HOCl near about 0.02 ppm, the level that reliably prevents algae in a balanced pool. With little or no CYA, a small FC is nearly all active HOCl, which is why a commercial pool runs FC 1–3.
In numbers
A "normal" 3 ppm at CYA 90 is below the 7.5% floor, so active HOCl is too low and algae grows.