Green Pool Rescue: fix it this weekend

Updated: October 13, 2026

A green pool is an algae bloom, and algae loses to exactly one thing: enough chlorine, held long enough. Here's the battle plan — with your exact dose computed below, not a scoop-per-10,000-gallons guess.

First, 60 seconds of triage

What you see	What it is	Your move
Green water, slick walls	Green algae bloom	This page — shock and hold
Milky white, not green	Cloudy water, not algae	Cloudy water guide
Yellow/brown dust on shady walls	Mustard algae	This plan, at a higher shock level (Trouble Free Pool uses 60% of CYA)
Clear but chlorine won't hold	Something consuming FC	Chlorine-not-holding guide

Step 1 — Test three things: FC, pH, CYA

CYA (stabilizer) decides everything. It binds most of your chlorine in reserve, so the free chlorine needed to kill algae is about 40% of your CYA reading. Then bring pH to 7.2–7.5 before you shock: chlorine works less well as pH rises, and the pH test stops reading correctly once free chlorine passes 10 ppm.

Step 2 — Shock to YOUR number, not a bag count

Enter your volume, current chlorine, and CYA. This is DoseMyPool's real engine:

Liquid chlorine is the cleanest tool for the job (adds nothing but salt). Cal-hypo works too, but it adds calcium — skip it if your calcium hardness is already high.

Step 3 — Brush everything, then filter around the clock

Chlorine can't kill what it can't touch. Brush the walls, floor, steps, and behind the ladder, then run the filter 24/7 until clear. Rising filter pressure is good news — it's catching dead algae. Backwash or rinse the cartridge when pressure climbs about 25% over clean.

Step 4 — Hold the line (this is where most people lose)

One shock dose doesn't end a bloom; algae regrows the moment chlorine sags. Test and top up back to shock level at least three or four times a day, but no more often than every two hours. The more often you can do it, the faster the pool clears.

Step 5 — Know when you're actually done

• Water is clear (you can see the main drain crisply)
• Combined chlorine 0.5 ppm or lower
• FC holds overnight — loses no more than ~1 ppm with the sun down

Then let FC drift down to its normal CYA-based target and get back to a weekly test rhythm — that's the part the app automates, with reminders, history, and exact maintenance doses.

Or let the app hold the line for you

This entire battle plan is built into DoseMyPool as guided Shock Mode: it computes your exact shock target from CYA, schedules the morning and evening re-test reminders, recalculates the dose from every new reading, tracks the overnight chlorine-loss test automatically, and only declares victory when all three finish criteria hold — with a Live Activity showing the rescue's progress on your Lock Screen the whole time. No spreadsheet, no guessing whether you're done.

FAQ

Sources

1. Trouble Free Pool, SLAM Process Checklist (V2). Shock at CYA ÷ 10 × 4, testing and topping up 3–4 times a day, no more often than every 2 hours, pH test unreliable above 10 ppm, cleaning the filter at 25% over clean pressure, replacing water above CYA 70, and the three finish criteria
2. Trouble Free Pool, CYA Chlorine Relationship (Chlorine/CYA chart). Shock level at 40% of CYA, and 60% for yellow or mustard algae
3. CDC, Home Pool and Hot Tub Water Treatment and Testing (May 10, 2024). Keeping pH in 7.0–7.8 balances killing germs against comfort and equipment
4. CDC, Model Aquatic Health Code, 5th edition (2024), Code Language. Sections 5.7.3.1.3.2 (90 ppm CYA cap) and 5.7.6.1 (bottom visible)

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