Why your pool went green
Cyanuric acid protects chlorine from sunlight, but it also weakens it. The more CYA you have, the more free chlorine you need to keep the same killing power. When free chlorine drifts below the minimum for your CYA, even for a day or two of hot weather, algae gets a foothold and the pool turns green fast.
That is why a single dose of shock often fails. The problem is not a lack of one big dose, it is that chlorine has not been held high enough, long enough. Know your CYA first; the right shock level depends on it.
Set your shock level from CYA
The shock (SLAM) level is about 40 percent of your CYA in free chlorine. Test your cyanuric acid, then use the shock calculator to get the exact target and how much chlorine to add to reach it. Use liquid chlorine or plain bleach, since cal-hypo adds calcium and dichlor adds even more CYA, which is the last thing a green pool needs.
If your CYA is very high, the shock level becomes hard to reach and hold, and the better move is to lower CYA first by draining and refilling some water. A pool with CYA of 100 needs a brutal amount of chlorine to shock.
Hold it, do not just dose once
This is the part people skip. Raise free chlorine to the shock level, then test and re-dose to bring it back up several times a day. Algae consumes chlorine quickly, so it will drop, and every time it drops you add more to return to the shock level. Keep the pump running the whole time and brush the walls and floor daily to break up algae and expose it.
Clean or backwash the filter often during this, because it fills with dead algae. A clogged filter stalls the whole process.
Know when you are done
The pool is clear of algae when three things are all true: the water is clear, combined chlorine is 0.5 ppm or less, and free chlorine drops no more than 1 ppm overnight. That overnight test is the real proof there is no algae left feeding on chlorine in the dark. Only stop when all three pass, or it comes back.
Once it passes, let chlorine drift back down to your normal target for your CYA and resume regular dosing with the chlorine calculator.
Keep it from coming back
Algae is a chlorine problem, so the prevention is simple: keep free chlorine in range for your CYA every day, not just when the water looks off. Test regularly, especially in hot weather and after heavy use or rain. If you keep ending up green, your CYA is probably too high, forcing a chlorine target you are not keeping up with.
Frequently asked questions
How do I get rid of algae in my pool?
Raise free chlorine to the shock level for your cyanuric acid, which is about 40 percent of CYA, and hold it there by retesting and re-dosing several times a day while the pump runs and you brush daily. Continue until the water is clear, combined chlorine is 0.5 ppm or less, and chlorine drops no more than 1 ppm overnight.
Why does my pool keep turning green?
Free chlorine is dropping too low for your cyanuric acid level, usually because CYA is high and the matching chlorine target is not being kept up. Test daily and keep chlorine in range, and if CYA is very high, lower it by partially draining and refilling so the chlorine target is realistic.
What chlorine should I use to clear a green pool?
Liquid chlorine or plain unscented bleach. They add no cyanuric acid or calcium, so they will not make the underlying problem worse. Avoid dichlor, which piles on CYA, and be careful with cal-hypo, which adds calcium hardness.
How long does it take to clear a green pool?
Anywhere from a couple of days for a light tinge to a week or more for a deep green or black-algae case. It depends on how high your CYA is and how consistently you hold the shock level. The water tells you when it is done, not the calendar.
Do I need algaecide to kill algae?
Usually no. Chlorine held at the right level kills algae on its own, and the SLAM method clears even bad blooms without algaecide. Algaecide can help prevent algae between treatments, but it is not how you clear an active green pool.