Why the pool turned green
Green means free chlorine hit zero long enough for algae to bloom — almost always because chlorine ran out, stabilizer (CYA) got so high the chlorine stopped working, or the filter/pump wasn't circulating. You won't keep it clear until you fix the cause, not just the symptom. The most common hidden culprit is sky-high CYA: if a pool eats chlorine no matter how much you add, test stabilizer before anything else.
Step 1 — Test before you touch it
Get real numbers, not a glance. You need:
- Free chlorine (FC) — will read at or near zero
- pH — chlorine works far better at the low end of the range
- CYA (stabilizer) — this sets how much chlorine you actually need
- Total alkalinity — for balance and to protect surfaces during the shock
Step 2 — Balance pH first
Lower pH to about 7.2 before shocking. Chlorine is dramatically more effective as a sanitizer at 7.2 than at 7.8 — same dose, far more killing power. Use the acid dose calculator to get there.
Step 3 — Shock to breakpoint and hold it
This is where green pools are won or lost. You're not adding a little chlorine — you're raising FC to shock level for the pool's CYA and holding it there until the algae is dead. As a rule of thumb, shock level is roughly 40% of the CYA reading (a pool at CYA 50 needs FC held near 20 ppm). Test and re-dose every few hours; algae devours chlorine, so one big dose that fades by morning won't do it.
The chlorine dosing calculator turns your current and target FC into an exact dose. Liquid chlorine is the pro choice here — it won't pile on more CYA (the thing that may have caused the problem) or calcium.
Step 4 — Run the filter 24/7 and brush
Dead algae has to be filtered out, so run the pump continuously — not on a timer — until the water clears. Brush the walls and floor daily to knock algae loose into the water where chlorine and the filter can reach it. Backwash or clean the filter as pressure climbs; a clogged filter is the second most common reason a “treated” pool stays cloudy.
Step 5 — Clear the cloudy stage
Once the algae is dead, green turns to cloudy gray/white — that's progress, not a setback. Keep the filter running and the water will clear over the next day or two. A clarifier or flocculant can speed it up on a heavy bloom. The pool is done when FC finally holds steady and the water is clear to the bottom.
How long does it take?
A light green pool with good circulation can clear in 24–48 hours. A deep-green or black-algae “swamp” can take 4–7 days and may need a partial drain if CYA is so high that shock level is impractical to reach. Set that expectation up front — it's the single biggest source of unhappy green-pool customers.
Pricing a green-pool recovery (for pros)
Never fold a recovery into the regular monthly rate — quote it as a job. You're buying a lot of chlorine, making repeat trips to re-dose, and carrying the risk that high CYA forces a drain. Price the chemicals, the multiple visits, and a margin for the uncertainty, and get the quote approved before you start. See the pricing guide for the per-job math, and log every dose so the recovery is documented if the customer questions the bill — Tadpole records readings and doses per visit.
Keeping it from coming back
- Fix the CYA — if stabilizer is above ~80–100, a partial drain is the only real cure.
- Hold a chlorine residual — never let FC hit zero again, especially in summer heat.
- Keep water moving — adequate daily pump run time (see the pump run time calculator).
- Brush weekly — algae starts in the spots water circulation misses.
