Why Is My Hot Tub Cloudy or Green? Step-by-Step Fix

A cloudy hot tub almost always means one of five things: low sanitizer, high pH or alkalinity, a dirty filter, used-up (combined) chlorine, or old water. To shock, use the amount on your product’s label, scaled to your tub. Example: a label that says 2 tbsp per 500 gallons means 1.6 tbsp for a 400-gallon tub. A green tub needs the same shock plus a filter clean, unless the color comes from metals.

5-question diagnosis

Test first, then answer these in order. The first "yes" is usually your cause.

Is sanitizer low? Free chlorine (FC) under 3 ppm, or bromine under 4 ppm. Germs grow fast in warm water and, with body oils, cloud it within a day or two.
Is pH above 7.8, or total alkalinity (TA) above 120 ppm? High pH makes calcium come out of the water as a fine haze. It also makes chlorine weaker.
When did you last clean the filter? If it has been more than a month, or the pleats look gray and matted, the filter can't catch fine particles.
Is combined chlorine (CC) 0.5 ppm or more? CC = total chlorine − free chlorine. High CC means waste from sweat, lotions, and urine has built up. It often comes with a "chlorine" smell.
How old is the water? After many soaks, dissolved solids build up, and chemicals can no longer keep the water clear. If it's been several months, or sanitizer disappears within hours in an unused tub (a sign of biofilm in the plumbing), plan to drain and refill.

Cloudy: sanitizer, pH/TA, filter, combined chlorine

Work through the fixes in this order, retesting after each:

Balance pH and TA: pH to 7.4–7.6 and TA to 80–120 ppm. See bringing high pH down if needed.
Clean the filter. Hose it out pleat by pleat. If it's been a while, soak it overnight in a filter cleaner, or swap in a spare. Filters wear out. If the fabric is torn or the end caps are cracked, replace it.
Shock. Use chlorine shock (not MPS) when the water is cloudy, because you need a sanitizer, not only an oxidizer. Use the amount on its label. The calculator at the top scales it to your tub.
Run the jets with the cover off for a few hours, then leave the pump on its normal filtration cycle overnight.
Retest in the morning. If the water is clear and FC holds at 3–4 ppm (or up to your label's limit), you're done. If it's still cloudy with good numbers, clean the filter again and consider the water's age.

Clarifier can help a filter catch fine particles, but only after the cause is fixed. Used alone, it only hides the problem for a day or two. Pools get cloudy for similar reasons. If you also have a pool, see cloudy pool water.

Green: algae or metals?

Algae or metals: how to tell
Clue	Points to algae	Points to metals
Feel	Slimy walls, seats, or filter	Smooth surfaces
Clarity	Cloudy green	Clear, tinted green or blue-green
Sanitizer	Near zero, won't hold	Holds normally
When it appeared	After days of low sanitizer or a warm, uncovered tub	Right after a shock, a refill from a well, or a long stretch of low pH

Algae: scrub every surface you can reach, clean or replace the filter, and shock with chlorine. Shock as your product’s label directs, with the cover off. Retest the next day and repeat only as the label allows, until the water is clear and chlorine holds overnight. Then drain, clean, and refill: algae hides in the plumbing and comes back if you only treat the water.

Metals: a green or blue tint in clear water usually means copper. Yellow or brown means iron. Low pH can dissolve copper from heater parts, and well water brings iron. More chlorine makes metal color worse, not better. Use a metal sequestrant as its label directs, fix pH, and consider a hose-end filter for the next fill.

Yellow water: in a tub that isn't slimy, yellow usually means iron from the fill water, pollen, or tannins from leaves. Test for iron if you can. For pollen, skim the water and clean the filter.

Shock dose for your tub

The calculator at the top uses your free and total chlorine. If combined chlorine is 0.5 ppm or more, it tells you to shock and scales the dose on your product’s label to your tub. With dichlor, each 10 ppm of chlorine also adds about 9 ppm of stabilizer (CYA). That doesn't matter if you plan to drain soon. If you don't, use bleach or liquid chlorine instead. Wait until FC is back at or below your label's limit (4 ppm on many labels) before soaking. More on shock types and timing: how to shock a hot tub. Always follow the product label.

When to drain and refill

Chemicals can't fix everything. Drain when:

the water stays dull or foamy after a shock and a clean filter (foam usually comes from soaps, lotions, and old water; see foamy hot tub);
sanitizer vanishes within hours in an unused tub, which points to biofilm in the pipes: run a line cleaner through the old water first, following its label;
you have used dichlor for months and CYA is high;
algae has taken hold, or metals keep staining;
it has been a long time. A common rule (gallons ÷ 3 ÷ daily soakers) gives the interval in days. We could not confirm its original source, so treat it as a rough guide. The water change calculator does the math.

Safety

Don't soak in cloudy or green water. Warm, poorly sanitized water can spread germs, including the bacteria that cause "hot tub rash".
Never mix shock, acid, clarifier, or line cleaner with each other. Add one product at a time.
Don't soak after shocking until free chlorine is back at or below the swim limit on your label (4 ppm on many labels).
If someone swallows spa chemicals or gets them in their eyes, call Poison Control at 1-800-222-1222.
Always follow the product label. It is a violation of federal law to use a pesticide product in a manner inconsistent with its labeling.

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