Green Pool to Clear: Fixing Algae in NY

You opened the cover, looked out the window after a long weekend, or came home from vacation — and your pool is green. Sometimes it's a light tint. Sometimes it's a swamp. Either way, every Westchester pool owner deals with at least one algae bloom in their ownership lifetime, and most deal with several.

Here is the exact green-to-clean process we run on Westchester pools, in the order it actually needs to happen. Skip steps and you'll be dumping chemicals into water that can't absorb them.

Why your pool turned green

Algae needs three things: sunlight, warm water, and a nutrient source (phosphates, nitrates, or organic debris). Westchester summers deliver all three. The bloom itself happens when the chlorine residual drops below 1 ppm for even 24–48 hours — usually after a thunderstorm, a heavy bather day, or a missed week of service.

Common Westchester triggers we see: a tripped GFCI that shut the pump off for two days, a clogged filter the homeowner didn't notice, a cyanuric acid level too high so the chlorine became inert, or simply a pool closed too late in fall with warm water under the cover.

Step 1 — Test before you treat

Take a water sample to a pool store or use a reliable home test kit. You're looking for:

• pH: needs to be 7.2 (lower than normal) so chlorine is more aggressive
• Total alkalinity: 80–120 ppm
• Cyanuric acid (CYA): under 50 ppm — high CYA is the #1 hidden cause of stubborn algae
• Phosphates: under 200 ppb if testable

If CYA is over 80, no amount of chlorine will kill the algae. You'll need to partially drain and refill to dilute it before shocking. This is the step that fails 80% of DIY green-to-clean attempts.

Step 2 — Brush everything

Algae cells protect themselves with a biofilm that chlorine can't penetrate. Brush the walls, floor, steps, and especially behind the ladder and under the skimmer mouths. You're physically breaking the film so chlorine can reach the cells. Use a stiff nylon brush for plaster, a softer brush for vinyl.

Step 3 — Adjust pH down

Lower pH to 7.2 with muriatic acid or dry acid. Chlorine is 3–4x more effective at killing algae at pH 7.2 than at 7.8. This single adjustment cuts chemical cost in half.

Step 4 — Triple shock

Use calcium hypochlorite (cal hypo) shock at 3x the normal dose. For a 20,000-gallon Westchester pool, that's roughly 6 lbs. Pre-dissolve in a bucket of water (never dump dry granules onto a vinyl liner — it bleaches), then pour around the perimeter with the pump running.

Run the pump 24 hours non-stop. Do not turn it off at night. Algae needs filtration to clear, and the pump is what delivers chlorinated water back to the pool.

Step 5 — Filter, backwash, repeat

Within 12–24 hours the green should shift to cloudy gray or white. That's dead algae suspended in the water. Now your filter has to do the work:

1. Sand filters: backwash every time pressure rises 8 PSI above clean baseline. Expect to backwash 3–5 times.
2. Cartridge filters: pull and hose them out every 12 hours. Have a spare set ready.
3. DE filters: backwash, then add fresh DE through the skimmer.

Add a water clarifier or flocculant after the second day to coagulate fine particles the filter can't catch on its own.

Step 6 — Final polish

By day 3–4, the water should be clear or near-clear. Vacuum the floor (to waste, not through the filter — that pushes dead algae back into circulation), brush one more time, and re-test all chemistry. Bring pH back to 7.4–7.6, chlorine to 2–3 ppm, and add an algaecide as preventative.

When DIY won't work

Call a pro if any of these are true:

• Water is so dark you can't see the second step
• Mustard or black algae on the walls (these need specialized treatment)
• CYA is over 100 and you can't drain easily
• You've shocked twice with no improvement
• Filter pressure won't drop below baseline after multiple backwashes

These cases need a service tech with the right chemicals, a portable filter, or in extreme cases a full drain and acid wash. We see these every June in Westchester — usually from pools that were opened a week too late.

Preventing the next bloom

Weekly service, balanced CYA, a working pump on a timer that runs 8–12 hours a day, and a 5-minute homeowner check during heat waves. Algae is almost always a chemistry problem disguised as a biology problem — get the chemistry right and blooms become rare.

Clear Pools & Hot Tubs offers same-week green-to-clean service across Westchester. Most pools are swim-ready in 3–5 days. Request a quote and we'll get you back in the water.