Why is my hot tub water cloudy, foamy, or green?
Each symptom points at a different cause. Read the water first, then fix in the right order.
Bad-looking spa water is almost always a chemistry story, and the symptom tells you most of the plot. Below are the four you'll meet most — what each one means, and what to check first. The recurring theme: low sanitizer and out-of-range pH cause more of these than anything else, so test before you reach for a bottle.
Cloudy water
What it is: hazy, milky, or dull water you can't quite see the bottom through.
Usual causes:
- Low sanitizer — not enough free chlorine/bromine to clear organic load; the most common cause.
- High pH or high calcium hardness — both let fine scale and particulates haze the water.
- Bather load — lotions, oils, and dead skin overwhelm the water after a busy soak.
- A clogged or tired filter that can't polish out the fine stuff.
Fix order: test and restore sanitizer → check pH and alkalinity are in range → rinse or replace the filter → shock if it persists. Give the filter time to clear it once the chemistry is right.
Foamy water
What it is: bubbles that pile up and linger instead of popping.
Usual causes: a build-up of organic residue — soaps, detergents from swimwear, body lotion, and the by-products of bather waste. Soft water (low calcium hardness) makes foam worse.
Fix: shock the water to burn off the organics, and check calcium hardness isn't too low. Anti-foam products are a cosmetic patch, not a cure — if foam keeps coming back, the water is overdue for a drain and refill. Rinsing swimwear in plain water before soaking helps a lot.
Green water
What it is: a green tint, sometimes slippery surfaces.
Two different causes — and they're fixed differently:
- Algae (cloudy green, slimy) — sanitizer dropped too low and algae took hold. Shock hard and brush.
- Dissolved metals (clear green, often after a fresh fill or after shocking) — copper or iron in the source water oxidizing. This one a shock won't fix; it needs a metal sequestrant and balanced pH.
Clear-but-green usually means metals; cloudy-and-green usually means algae. Telling them apart saves you from shocking water that needs sequestering instead.
Smelly water
What it is: a strong "chlorine" smell, or a musty/eggy odor.
Cause: counterintuitively, a strong chlorine smell usually means too little free chlorine, not too much — the odor is chloramines (combined chlorine), the spent by-product. A musty smell points at biofilm in the plumbing.
Fix: shock to break the chloramines (not skip the chlorine), and consider a pipe/line flush if the musty smell persists through a refill.
Every one of these can be made worse by guessing. A reading of free chlorine, pH, and total alkalinity tells you which story you're in before you add a single chemical — and stops the classic mistake of shocking green water that actually needs a metal sequestrant.
The one-line diagnostic
| Symptom | Check first |
|---|---|
| Cloudy | Sanitizer, then pH / filter |
| Foamy | Organics / low calcium → shock |
| Green & cloudy | Algae → shock hard |
| Green & clear | Metals → sequestrant |
| Strong chlorine smell | Chloramines → shock |
Gear this guide uses
The few things that clear up each symptom — once a test tells you which story you're in.
- Every fix here starts with a reading, so keep spa test strips on hand before you reach for a bottle.
- Cloudy, foamy, and chloramine-smelling water all respond to spa shock to burn off the organic load.
- Clear-but-green metal staining needs a metal sequestrant, not more shock.
- If foam keeps returning, a defoamer is a cosmetic patch — fine in a pinch, but a drain-and-refill is the real cure.
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Sources
General guidance, not a substitute for your own testing. Causes overlap — confirm with a test before treating, and re-test after.