Hot tub algae
Quick answer
Algae in a hot tub means the sanitizer crashed and let it bloom. Clear it by balancing pH, then a heavy shock, brushing the walls and jets, running the filtration, and cleaning the filter afterward. A bad bloom is faster to beat with a drain and clean. Prevent the next one by holding a steady sanitizer level.
Algae is rarer in hot tubs than pools because the heat and sanitizer usually hold it off, so a green or mustard tinge is really a warning that sanitizer fell behind. Kill the algae, then fix why it ran low.
How to clear it
- Balance pH first. Bring pH to 7.2 to 7.8 so the shock works at full strength.
- Shock hard. Hit it with a strong chlorine shock; algae needs a bigger dose than a routine oxidation. See how to shock a hot tub and size the dose.
- Brush every surface. Scrub the walls, footwell, and around the jets to break the algae loose so the sanitizer and filter can get it.
- Run the filtration continuously to pull the dead algae out, and keep the chlorine level up until the water clears.
- Clean the filter afterward, since it is now full of dead algae. A heavy bloom can clog a cartridge enough to need replacing. See how to clean a hot tub.
For a heavy or slimy bloom, it is often faster and more certain to drain, scrub the empty shell, clean or replace the filter, and refill than to chase it in the water.
Add in small steps with the pump running, wait, then retest before adding more. Never mix chemicals together, and always follow your product label, which wins over any calculator.
Keep it from coming back
- Hold a steady sanitizer residual; algae only blooms when it drops. Test a couple of times a week.
- Shock weekly, and after heavy use, to clear the load before it feeds anything.
- Keep the filter clean and the cover on, since sunlight and debris both help algae start.
What helps
A strong chlorine shock to kill the bloom, and a fresh filter cartridge for after, since a heavy bloom can ruin the old one.
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Common questions
Is algae in a hot tub dangerous?
The algae itself is mostly a sign of trouble rather than the main hazard: water that let algae grow has had little working sanitizer, so it can also harbor bacteria. Do not soak until you have shocked it, cleared the algae, and confirmed a sanitizer reading back in range.
What is the yellow or mustard stuff, not green?
Mustard algae is a more stubborn type that clings and resists a normal sanitizer level. Treat it the same way but more aggressively: a heavier shock, thorough brushing, a filter clean or replacement, and in tough cases a full drain and scrub.
How do I stop algae coming back?
Keep a steady sanitizer residual, which is what normally keeps a hot tub algae-free, and shock regularly. Algae only gets a foothold when sanitizer drops, so the fix and the prevention are the same: do not let it run low.