Why is my hot tub water cloudy, foamy, or green?
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 |
Soak reads your numbers and tells you the fix
Enter your test readings and Soak tells you what's off, the exact dose to correct it for your tub volume, and the order to do it in — with guardrails that stop you over-dosing. Pay once, no subscription, no ads, fully offline.
Sources
- TroubleFreePool — cloudy water and algae troubleshooting
- Industry spa-care guidance on foam (organics / low calcium), chloramine odor, and metal staining
General guidance, not a substitute for your own testing. Causes overlap — confirm with a test before treating, and re-test after.