Green hot tub water: causes and how to fix it

Green hot tub water has two causes. If the water is cloudy green and surfaces feel slimy, it is algae from low sanitizer. If the water is clear green, especially right after a fill or a shock, it is dissolved metal, usually copper or iron. Algae is fixed with a chlorine shock, filter cleaning, and often a drain. Metals are fixed with a metal sequestrant and correct pH.

Algae or metals? How to tell

Sign	Algae	Metals
Clarity	Cloudy, murky	Clear, tinted like mouthwash
Surfaces	Slippery shell, slimy filter	Normal feel. Possible stains.
Sanitizer reading	Zero or very low	Can be normal or high
When it appeared	After days of neglect, cover left off, warm weather	Right after filling, or minutes after adding chlorine
Smell	Musty or swampy	None
Fill source	Any	Often well water, or old copper pipes

Fixing green water from algae

Algae needs light, warmth, and no sanitizer. A covered hot tub rarely grows it unless the sanitizer has sat at zero for days.

1. Keep everyone out. Low sanitizer means bacteria too.
2. Test and balance. Bring pH to 7.2–7.6 so chlorine works at full strength.
3. Remove the filter and clean it. Turn off power at the breaker first. Soak it in filter cleaner, or replace it if it is over a year old. Algae lives in the pleats.
4. Shock with chlorine. Use a spa chlorine shock per the label, jets on, cover off. See how to shock a hot tub.
5. Wipe the shell at and above the waterline with a clean cloth while the jets run.
6. Run the filter for several cycles, then rinse it again.
7. Retest the next day. The water should be clearing and holding chlorine. If chlorine is back to zero or the water is still green, drain.

For anything beyond a light tint, the dependable fix is to add a line flush product, circulate it per the label, drain the tub, wipe down the shell, and refill. Treating heavily contaminated water costs more in chemicals than new water does.

Fixing green water from metals

Copper and iron arrive in fill water or come from inside the tub when acidic water (pH under 7.0) corrodes a heater or other metal parts. Chlorine oxidizes the metal and the color shows up.

1. Test pH and alkalinity. Bring pH into 7.2–7.6 and alkalinity to 80–120 ppm.
2. Add a metal sequestrant (often sold as stain and scale control or metal remover) per the label for your tub's volume. It binds the metal so the filter can collect it.
3. Run the pump for several hours.
4. Clean the filter. The metal ends up there. Rinse it well, or soak it in filter cleaner.
5. Hold off on shock for a day. Heavy chlorine can break the sequestrant's hold and bring the color back.

If the tint returns each week, the metal level is high. Drain and refill through a hose pre-filter, and add sequestrant as the tub fills.

Where is the copper coming from?

If your fill water is clean and the tub still goes green, check your pH history. Water that sat below 7.0 can dissolve copper from older heater parts and fittings. Keeping pH and alkalinity in range stops it. See pH too high or too low. Some mineral cartridges and copper-based algaecides also add copper.

Other colors, briefly

• Brown or rusty: iron. Treat as metals.
• Yellow: pollen, low-level iron, or mustard algae. Check sanitizer first.
• Blue-green stains on the shell: copper that has dropped out of the water. Use a sequestrant and keep pH from going high.

Preventing green water

• Keep free chlorine at 1–3 ppm or bromine at 3–5 ppm at all times, including when you are away. A floater or a neighbor helps on long trips.
• Test 2 to 3 times a week.
• Shock weekly.
• Keep the cover on. No light, no algae.
• Rinse the filter every 1 to 2 weeks.
• If you are on a well, fill through a pre-filter and add sequestrant at every fill.
• Keep pH at 7.2–7.8 so the water does not corrode metal parts.

Green water that also turns hazy is covered in cloudy hot tub water. The same test, shock, and filter routine applies.