Green Hot Tub Water: Two Causes, Two Fixes

Quick answerGreen water is either algae (water looks green and feels slimy; sanitizer reads zero) or dissolved copper (water is clear but tinted green, often right after a refill). Algae: balance, double-shock, run the filter, and clean it after. Copper: add a metal sequestrant — do not shock, which can make metal staining worse.

Tell the two apart in 30 seconds

Scoop water into a white cup. Cloudy green with particles or slime on surfaces = algae. Clear but tinted green = metals, almost always copper from source water or corroded heater components. Algae shows up after sanitizer lapses (a vacation, a dead float). Metal tint shows up within a day of a fresh fill.

Fix algae

  1. Remove the filter and rinse it — it's contaminated.
  2. Balance pH to 7.2–7.4 (low end makes sanitizer more effective).
  3. Shock with dichlor at roughly double the normal dose. Bromine tubs: shock with MPS or chlorine — it regenerates bromine from the bank.
  4. Run the pump on high 4–6 hours, then recheck sanitizer and repeat if it fell to zero again.
  5. Once water clears, soak the filter in cleaner overnight or replace it.
  6. If algae recurs, drain, wipe the shell with diluted bleach solution, flush the lines with a plumbing purge, and refill.

Fix metal tint

  1. Add a metal sequestrant per label; it binds copper so it can't tint or stain.
  2. Run the filter continuously for 24 hours.
  3. Hold off on shock for a day or two — strong oxidizer plus dissolved copper is how tubs get green hair and black-green stains.
  4. If your fill water is well water, pre-filter future fills with a hose-end filter.
Green water with a strong musty smell and zero sanitizer for an extended period is a drain-and-refill situation. Don't spend a week chasing it chemically.

What you'll need

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Common questions

Is green hot tub water dangerous?

Algae itself is mostly a nuisance, but water that supports algae has had no effective sanitizer, so bacteria like pseudomonas may also be present. Don't soak until sanitizer holds a normal reading for 24 hours.

Why did my hot tub turn green right after refilling?

That's dissolved copper or iron in your fill water, not algae. Add a metal sequestrant and use a hose-end pre-filter on future fills.

Will shock alone fix green water?

For algae, shock plus aggressive filtration usually works. For metal tint, shock makes it worse — sequester the metals instead.

Want exact doses for your tub's volume and sanitizer system? Use the free Spa Assistant — test, enter your readings, get a dosing plan.

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