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Hot Tub Green Water: How to Fix Algae & Get Crystal Clear Water Again

Opened your hot tub cover to find green, murky water? You're not alone — it's the most common problem we hear about, especially in April and May when Bournemouth hot tub owners are starting up after winter or dealing with the first warm spells of the year.

Green water is almost always algae, and the good news is it's fixable. This guide covers exactly what's causing it, how to fix it step-by-step, and — most importantly — how to stop it coming back.

What Causes Green Hot Tub Water?

Understanding the cause helps you prevent it happening again. There are three main culprits:

1. Algae Growth (Most Common — 90% of Cases)

Algae spores are everywhere — in the air, on your skin, in tap water. Normally, your sanitiser (chlorine or bromine) kills them before they can multiply. But when sanitiser levels drop below effective levels, algae takes hold fast — sometimes overnight in warm conditions.

Common triggers in the Bournemouth area:

• After winter storage — tubs that have been sitting unused, even with a cover, often develop stagnant water that goes green within days of being refilled if not treated immediately
• Warm weather + sunshine — UV breaks down chlorine rapidly. A sunny April day in Bournemouth can halve your chlorine levels in a few hours
• Heavy use without re-dosing — a family hot tub session introduces body oils, sweat, and skin cells that consume sanitiser faster than normal
• Forgetting to dose for a few days — life gets busy, sanitiser drops to zero, algae moves in
• Running without a filter — even briefly, unfiltered water allows particles to circulate and algae to establish

2. Mineral Contamination (Less Common)

Sometimes green water isn't algae at all — it's dissolved metals in your water supply reacting with chlorine or pH changes. Copper is the usual culprit, and Bournemouth's water supply (Wessex Water, moderately hard at ~200mg/L CaCO3) can occasionally contain trace metals, particularly if your home has older copper pipework.

How to tell the difference: Metal-caused green water is usually a clear green tint (you can see through it). Algae-caused green water is cloudy and opaque. If the walls aren't slimy and the water is clear-but-green, metals are likely.

3. Pollen & Organic Debris

In spring, pollen from trees and gardens can give water a yellowish-green tinge. This isn't harmful in the same way as algae, but it does consume sanitiser and can lead to algae if not managed. Common in Bournemouth with the pine trees across the heathland areas (Talbot Heath, Meyrick Park).

How to Fix Green Hot Tub Water: Step-by-Step

Option A: Shock Treatment (For Mild Cases)

If the water is slightly green but walls aren't slimy, try shock treatment first:

1. Remove and clean your filter — if it's heavily soiled, replace it. A clogged filter can't remove dead algae after treatment. For Lay-Z-Spa users, a new VI filter cartridge is about £5 for a twin pack
2. Test your water — check pH, alkalinity, and sanitiser levels with test strips. Adjust pH to 7.2-7.6 first (shock treatment works poorly outside this range)
3. Add chlorine shock treatment — use calcium hypochlorite granules (not non-chlorine shock — it won't kill algae). Dose at 2x the normal rate. Pre-dissolve in a jug of warm water before adding to the tub
4. Add algaecide — a dedicated hot tub algaecide (like Clearwater Algaecide, available at B&Q, Amazon, or Lay-Z-Spa.co.uk) provides a secondary kill and prevents regrowth
5. Run the pump continuously for 24 hours — the filter needs to circulate and capture dead algae. Leave the cover off for the first hour to let chlorine gas dissipate
6. After 24 hours, retest and adjust — if water is clearing, maintain chlorine at 3-5mg/L and run the pump for another 24 hours. Clean or replace the filter again (it'll be catching a lot of debris)
7. Add clarifier — once the green is fading, a water clarifier helps the filter capture the remaining fine particles faster

Option B: Full Drain & Clean (For Severe Cases)

If the walls are slimy, the water is dark green, or you can't see the bottom, shock treatment alone won't cut it — the biofilm on the surfaces needs physically removing. This is also the right approach if you're starting up after winter and found green water in an un-drained tub.

1. Drain the tub completely — use the drain valve (not the pump). For Lay-Z-Spa, connect the drain cap and garden hose to direct water away from the garden (heavily chlorinated water kills plants)
2. Scrub the interior — use a soft cloth or sponge with diluted white vinegar (50:50 with water). This kills remaining algae and is safe on PVC liners. Don't use bleach on inflatable tubs — it degrades the material over time
3. Clean the pump housing — remove the filter, rinse the housing, check the impeller isn't clogged with debris
4. Flush the plumbing — for hardshell tubs, run a pipe flush product before draining. For inflatables, run clean water through the pump briefly
5. Refill with fresh water — use a hose filter if you have hard water (reduces minerals that can cause green tinting). In Bournemouth, a hose filter is a worthwhile investment at around £20
6. Heat and dose immediately — add chlorine granules before the water reaches temperature. Algae can start growing in untreated water within hours in warm weather
7. Install a fresh filter — always start with a clean or new filter after a full drain

Bournemouth-Specific Tips

A few things specific to running a hot tub in the BCP area:

• Hard water scale — Wessex Water supplies moderately hard water (typically 200-250mg/L CaCO3). This causes limescale build-up on the heating element and inside pipes, reducing heating efficiency and potentially triggering E02 errors. Use a scale preventer (like No Scale) added weekly
• Coastal salt air — if your tub is in a garden close to the coast (Southbourne, Boscombe, Westbourne), salt air can corrode metal fittings faster than inland. Rinse the pump exterior with fresh water monthly
• Spring pollen season — April/May brings heavy tree pollen across the heathland areas. If your tub is near trees, check and clean your filter weekly rather than fortnightly during pollen season
• Hedgehog season — from April onwards, hedgehogs are active. If your tub is ground-level, ensure hedgehogs can't climb in (they can climb!). A simple step or ramp near the tub helps them escape if they fall in

When Green Water Means a Bigger Problem

Sometimes green water is a symptom of a mechanical issue, not just a chemical one. If the water keeps going green despite proper treatment, check for:

Pump Not Circulating Properly

If the pump is running but water isn't actually flowing through the filter, chemicals can't do their job. You might hear the pump running but see no water movement. This is often an impeller problem — the part that pushes water through the system. See our E02 error code guide for diagnosis.

Heater Not Working

Cold water + chemicals = slow chemical reaction. If your tub isn't reaching temperature, sanitiser is less effective. If the water isn't heating, check our hot tub not heating guide.

Filter Housing Cracked or Misaligned

On Lay-Z-Spa models, the filter housing can crack or the filter can sit incorrectly, allowing unfiltered water to bypass the cartridge entirely. Visually inspect the housing for cracks and ensure the filter sits flush.

Persistent Biofilm in Plumbing

If you've cleaned the tub, refilled with fresh water, and it goes green again within 48 hours, there's likely biofilm established inside the plumbing that you can't reach with a cloth. This needs a dedicated pipe flush chemical — fill the tub, add the pipe flush, run the pump for 30 minutes, drain, rinse, and refill.

Prevention: Keeping Your Water Crystal Clear

Once you've cleared the green water, these habits stop it coming back:

Task	Frequency	Why It Matters
Test sanitiser & pH levels	2-3 times per week	Catches drops before algae can establish
Add chlorine/bromine	After every use + twice weekly	Maintains kill levels for algae and bacteria
Run pump for 2+ hours	Daily	Circulates chemicals and filters debris
Clean/replace filter	Every 2-4 weeks (weekly in spring)	Clogged filter = poor circulation = algae
Shock treatment	Weekly or after heavy use	Kills organisms regular dosing misses
Add algaecide	Weekly	Secondary defence against algae growth
Full water change	Every 2-3 months	Resets dissolved solids that build up over time
Shower before use	Every time	Removes body oils, sun cream, and products that consume sanitiser

Products You'll Need

All available at B&Q Bournemouth, Amazon, or directly from Lay-Z-Spa.co.uk:

• Chlorine granules (calcium hypochlorite) — for regular dosing and shock treatment (~£8-12 for 1kg)
• Test strips — 6-in-1 strips test chlorine, bromine, pH, alkalinity, hardness (~£8 for 50 strips)
• Algaecide — Clearwater Algaecide or similar (~£6-8 for 1L)
• Water clarifier — helps filter capture fine particles after treatment (~£5-7)
• Scale preventer — essential in Bournemouth's hard water area (~£7-10)
• Filter cartridges — keep 2-3 spare. Lay-Z-Spa VI cartridges: ~£5 for twin pack, ~£12 for six
• Hose filter — pre-filters fill water to remove minerals and chloramine (~£20, reusable)

Frequently Asked Questions