Green hot tub water: why it happens and what to do

Algae or metals: how to tell

Test the water first. A low or zero sanitiser reading with cloudy green water and a slimy liner points strongly to algae. Clear water with a green tint and normal sanitiser, especially just after filling, points to metals.

Fixing algae

On an inflatable tub with only a few hundred litres, a drain and refill is often quicker and simpler than battling a heavy algae bloom. Clean the liner with a product suitable for hot tubs, rinse well and fit a new filter before refilling. See how to drain an inflatable hot tub.

1. 1

   Remove the filter and clean it thoroughly, or replace it if it is heavily soiled.

2. 2

   Test and bring alkalinity and pH into range.

3. 3

   Shock with a chlorine shock at the dose on the label for your tub's litres.

4. 4

   Run the filter pump with the cover open for the time on the label.

5. 5

   Brush or wipe the liner, seams and waterline to loosen algae.

6. 6

   Retest the next day; shock again if the label allows and the water is still green.

7. 7

   If the water does not clear, drain, clean the liner and refill.

Fixing metals in the water

Some fill water contains small amounts of copper or iron, from old pipes or private supplies. When chlorine oxidises them, the water can turn green, brown or yellow, and stains may appear. A metal remover or sequestrant, used as the label directs, binds the metals so the filter can catch them or keeps them in solution.

Keep pH in range, because high pH makes metal staining more likely. If the problem comes back after every refill, a hose end pre-filter when filling can help. Replace the filter cartridge after treating metals, as it will have collected them.

Why it happens

Algae needs light, warmth and nutrients, and a hot tub with low sanitiser provides all three. It often follows a busy weekend, a few days without testing, or leaving the cover off in sunshine. Leaves, pollen and garden debris add nutrients. A dirty filter makes it worse by lowering flow and holding organic matter.

Metals come from the fill water itself, so they are more common in some areas and with private water supplies.

Preventing green water

A steady routine is the best prevention. See what chemicals a hot tub needs for a simple weekly plan.

• Test a few times a week and keep sanitiser in range.
• Top up sanitiser after every use.
• Shock weekly or after a busy session.
• Rinse the filter every few days and replace it when worn.
• Keep the cover on between uses, especially in sunshine.
• Drain and refill as often as your manual suggests.

Note:Do not use the tub while the water is green or until sanitiser is back in range. Never mix chemicals, wear gloves if the label advises it and keep products away from children.

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After the water clears

Once the water is clear and readings are back in range, keep a close eye on it for the next week. Test daily, top up sanitiser after every use and rinse the filter every couple of days, because any algae left in the pipes or seams can regrow quickly. A second shock a few days later, as the label allows, helps finish the job.

Check the underside of the cover too. Algae and grime there drip back into the water every time the lid is lifted. Wipe it with a cleaner made for hot tub covers, rinse and let it dry in the air before refitting.

Frequently asked questions

Why does my hot tub turn green so quickly? Usually because sanitiser ran out in warm weather. Keep it topped up and the cover on.

Can I use a green hot tub? No. Wait until it is clear and readings are back in range.

Should I drain a green hot tub? For heavy algae in an inflatable tub, a drain, clean and refill is often quickest.

Why is my water green but clear? That points to metals rather than algae. Use a metal remover as the label directs.