Green Hot Tub Water — Causes and How to Fix It

Green water in a hot tub is alarming — but it's more common than you might think, particularly in summer or after a period of neglect. Here's what's causing it and what to do.

Why has the water turned green?

In almost every case, green water is caused by algae growth. Algae multiply rapidly when sanitiser levels drop, particularly in warm weather. A few days with no chlorine and some sunlight is enough for a hot tub to turn green. Less commonly, a copper corrosion issue or certain algaecide chemicals can cause greenish discolouration without algae being present.

Is it safe to use a hot tub with green water?

No. Green water indicates bacterial contamination that your sanitiser is no longer controlling. The same conditions that allow algae to grow also allow potentially harmful bacteria (including Legionella in warm water) to proliferate. Do not use the tub until the water is clear and chemistry is balanced.

How to fix green water

1. Test the water — confirm pH and alkalinity before shocking
2. Adjust pH to 7.2–7.4 — lower pH makes chlorine more effective
3. Superchlorinate — add a large dose of chlorine shock (granular chlorine or liquid chlorine). For a 1,500 litre tub, use 3–4 times the normal dose
4. Run the jets for 30–60 minutes to circulate the shock treatment
5. Add algaecide if algae is persistent
6. Leave overnight — the dead algae will cloud the water further before clearing
7. Clean or replace the filter — dead algae will clog it quickly
8. Test and rebalance — check chemistry is correct before using the tub

What if the shock doesn't work?

If the water is still green after 24 hours of treatment, the algae load is too high and the most efficient fix is to drain, scrub the shell, and refill. This is always faster than fighting a severe algae bloom with chemicals. See our drain and refill guide.

Could it be copper rather than algae?

If the water is a clear blue-green (rather than murky green) and chemistry has been fine, it may be copper leaching from a heat exchanger or ioniser. This is more common with heat pumps and ioniser systems. Test for copper — if elevated, use a metal sequestrant and investigate the source.

Preventing a recurrence

• Never let free chlorine drop below 2 ppm
• Test the water at least twice a week in summer
• Keep the cover on when not in use — sunlight degrades chlorine rapidly
• Use a stabiliser (cyanuric acid) if the tub is in direct sunlight