Pink slime in a hot tub and how to stop it returning

Pink slime in a hot tub is a bacterial biofilm, most often linked to bacteria such as Serratia marcescens or Methylobacterium, not a type of algae. Shocking the water rarely clears it on its own. To stop it returning, purge the pipes, clean the filters and the underside of the cover, refill, and keep sanitiser steady.

The pink film you see on the waterline or in the filter housing is only the visible part. Most of the colony lives inside the plumbing, where shock cannot reach it properly. The sequence below deals with every hiding place in one go, so you only need to do the job once.

What is pink slime in a hot tub?

Pink slime is a slippery pink, orange or reddish film. It appears where water sits still or splashes and dries: the waterline, the filter housing, around the jets, under the headrests and on the underside of the cover. It feels slimy rather than gritty, and it wipes off easily before coming back within days.

The colour comes from bacteria. The species most often named are Serratia marcescens, which makes a red pigment, and Methylobacterium, a pink bacterium that tolerates chlorine fairly well. Without a laboratory test you cannot know which one you have, and you do not need to, because the treatment is the same.

The red pigment made by Serratia marcescens is called prodigiosin. The bacterium itself is a gram-negative, rod-shaped species that is common in damp places around the home, such as showers and sink drains.

Why pink slime is not algae

Many people call it pink algae, but the name is misleading. Algae are plant-like organisms that need light, and in hot tubs they usually show up as green or yellow growth. Pink slime grows well in the dark inside pipes, so it is clearly not algae.

This difference affects what you use against it. Algaecides do little against bacterial biofilm. What works is physical cleaning, a dedicated pipe purge product, and a steady sanitiser level afterwards.

What biofilm is and why it protects bacteria

Biofilm is a community of bacteria that stick to a surface and surround themselves with a slimy layer they produce. That layer shields the bacteria underneath from sanitiser. Our guide on how to get rid of biofilm in a hot tub covers biofilm in general, and pink slime is one visible form of it.

Biofilm feeds on what bathers leave behind: body oils, sweat, skin cells, lotions, make-up and detergent left in swimwear. Warm water, usually between 37°C and 40°C (99°F to 104°F), helps the bacteria grow faster.

Sunscreen is one of the most common sources of this food, especially in summer. Ask everyone to shower off sunscreen before they get in, because it leaves an oily film in the water that pink slime bacteria can feed on.

Where pink slime hides

• Inside the jet pipes and air channels, where water sits still when the pumps are off
• Deep in the pleats of the filter cartridges
• In the filter housing, the skimmer and behind the skimmer flap
• On the underside of the cover, especially along seams and the central fold
• Under and behind headrests, in cup holders and around steps
• Along the waterline, where oils collect
• Inside the garden hose you use to fill the tub

If you clean only the shell you can see, the bacteria left in these places will spread back into the fresh water. Within a few weeks the pink film is back.

Rainwater is another way in. Water that collects on top of the cover can carry bacteria, leaves and other organic matter, and it runs straight into the tub when you lift the cover off. Tip or brush the water off the cover before you open it.

Pink slime and white water mould

White water mould often appears alongside pink slime. It looks like a white, filmy or tissue-like deposit, and it is often caused by Fusarium species of fungi. The same purge, clean and shock sequence below deals with both, so you do not need a separate treatment.

The two problems support each other. White water mould forms a protective structure, and the pink bacteria colonise it and shelter inside it. If you see either one, treat the tub for both, because clearing only one leaves a home for the other.

Why does shocking alone rarely clear pink slime?

Shocking raises sanitiser to a high level for a short time. It kills bacteria floating in the water, and the slime on the shell may fade for a while. The bacteria protected inside the biofilm in the pipes and filter pleats mostly survive.

Once the shock wears off and sanitiser returns to normal, those survivors start to grow again. That is why the pink film often returns within a week or two of a shock. Shock is still part of the fix, but only as one step. Our guide on how to shock a hot tub, step by step explains the dose and timing.

An established biofilm also uses up sanitiser. If pink slime keeps returning, you will often find free chlorine falling below 1 ppm between doses, even though you add chlorine regularly. That pattern is a strong sign that biofilm is living in the plumbing.

Non-chlorine shock (potassium monopersulfate) is an oxidiser, not a sanitiser. It breaks down organic waste but does not kill bacteria the way chlorine does. For pink slime, a chlorine shock is the usual choice, including in many bromine tubs, but check your manual first.

What do you need before you start?

Set aside most of a day, including time to refill and reheat. Have everything ready before you drain, because many manufacturers advise against leaving an empty shell standing for long, especially in direct sun.

Expect the hands-on work to take about 4 to 6 hours, from the start of the pipe purge to a refilled tub. That estimate does not include the overnight filter soak or the time the fresh water needs to reheat, so plan for the tub to be out of use until the next day.

Draining through the drain valve usually takes 30 to 60 minutes, or about 10 to 15 minutes with a submersible pump. Allow 1 to 3 hours to refill a typical tub, depending on your water pressure and hose size.

• Test strips or a test kit for sanitiser, pH and total alkalinity
• A digital pH meter, if you prefer it to strips or a test kit for checking pH
• A hot tub pipe purge product (often sold as a line flush or pipe cleaner)
• Chlorine shock, such as sodium dichlor granules
• Filter cleaner and a bucket or container large enough to soak the cartridges
• A spare clean set of filters, if you have one
• A non-abrasive hot tub surface cleaner, soft cloths and sponges
• A soft brush or old toothbrush for jets and corners
• A submersible pump or wet and dry vacuum for draining
• Rubber gloves and eye protection
• A garden hose, with a hose pre-filter if you use one

Our guide to hot tub cleaning products and where each one goes explains which cleaner suits which surface. Avoid household cleaners on the shell or in the water. Many foam heavily or leave residues that feed the next layer of biofilm.

Safety: never mix chemicals, and never pour water onto chemicals. Add each product to the tub separately, following its label, and store chlorine products away from acids. Wear gloves and eye protection when handling shock and pipe purge, and switch the tub off at the isolator before you drain it or work near the equipment.

How do you get rid of pink slime in a hot tub, step by step?

Work through the stages in this order. The pipe purge comes first because it needs water circulating through the plumbing. Once you drain the tub, that chance is gone until the next refill.

Stage 1: purge the pipes

1. Test the existing water and bring pH roughly into the 7.2-7.6 range. Most purge products work best in balanced water.
2. Make sure the water is at its normal operating temperature, around 37°C to 40°C, before you add the purge product. Warm water helps the product work into the biofilm on the pipe walls.
3. Remove the filters or leave them in, as the product label instructs. Labels differ, and some suggest using an old filter to catch the debris.
4. Add the purge product at the dose on the label for your water volume. Many tubs hold between 1,000 and 1,500 litres (about 260 to 400 US gallons), but check your manual for the exact figure.
5. As a rough guide to quantities, many pipe purge products are dosed at about 30 to 60 ml per 1,000 litres of water (roughly 1 to 2 US fl oz per 250 US gallons). The label dose for your product always takes priority over this figure.
6. Once the purge product is in, add no other chemicals at all, including pH or alkalinity balancers, unless the purge label allows it. Other products can interfere with how the purge works.
7. Run all the jets on high, with the air controls open, for the time given on the label. Some products work in under an hour, others over several hours or overnight.
8. Watch the water. Brown, grey or pink foam and flakes show that the product is lifting biofilm out of the pipes. Scoop away heavy foam with a net if it starts to overflow.
9. Use the amount of foam as a rough guide. A little foam means the pipes were fairly clean, while heavy foam means a heavy build-up of biofilm was hiding in the plumbing.
10. Add chlorine shock during the purge only if the purge product label says this is allowed.

Stage 2: drain and clean every surface

1. Do not rinse the tub between the purge and the drain. Let the loosened biofilm leave with the purge water, so it does not settle back on the shell.
2. Switch off at the isolator, then drain the tub fully using the drain valve or a submersible pump.
3. Remove the sludge left in the footwell and seats with a wet and dry vacuum or old towels.
4. Once the tub is empty, give the shell a quick rinse with the hose to wash away loose purge debris, and remove that rinse water before you start scrubbing.
5. Clean the shell with a non-abrasive hot tub surface cleaner. Pay attention to the waterline, the seats, the steps and every corner.
6. Work one section at a time, scrubbing each section in circular motions to break up the film, and rinse each section before the cleaner dries. Do not let surface cleaner dry on the shell.
7. Scrub around each jet face, the filter housing, the skimmer flap and the cup holders with a soft brush.
8. If your jets can be removed, take them out and scrub the sockets behind them with a small brush. Biofilm builds up behind the jet face, where you cannot see it.
9. Lift out the headrests if they are removable. Clean both the pillow and the recess behind it.
10. Rinse the whole shell with clean water and remove the rinse water. Leftover cleaner can cause foam in the fresh water.

For a fuller walk-through of the shell and fittings, see our guide on how to deep clean a hot tub at water change.

Stage 3: clean the filters

Filters are often where pink slime looks worst. A quick rinse removes loose debris, but it does not remove biofilm from deep inside the pleats.

1. Hose each cartridge from top to bottom at an angle, opening the pleats as you go.
2. Soak the cartridges in filter cleaner for at least 8 hours, and ideally overnight, unless the label gives a different time.
3. After the soak, rinse each cartridge from the inside outwards through the pleats. This pushes loosened debris out of the cartridge rather than driving it deeper in.
4. Rinse thoroughly until no cleaner or foam remains.
5. Let the cartridges dry completely before you refit them. Full drying also helps kill any bacteria that remain.
6. Check each cartridge. Replace it if the pleats are torn or flattened, the end caps are cracked, or the pink colour returns soon after cleaning.
7. Replace any cartridge that is discoloured brown or grey all the way through, because it cannot be fully cleaned.

Many manufacturers suggest replacing cartridges every 12 to 24 months, depending on use, so check your manual. Our guide on how to clean a hot tub filter properly goes into more detail. Keeping two sets lets you rotate them, so one is always dry and ready.

Stage 4: clean the underside of the cover

The underside of the cover is warm, damp and dark, and it is easy to forget. Any pink slime that survives here drips straight back into the fresh water.

1. Take the cover off and place it with the underside facing up on a clean surface.
2. Wash the underside with a cleaner made for spa covers. Some cover makers accept a weak, diluted bleach solution and others do not, so check their care instructions first. Where bleach is allowed, a typical dilution is about 1 tablespoon (15 ml) of household bleach to roughly 4 litres (about 1 US gallon) of water.
3. Pay extra attention to the seams, the central fold and the edges, where slime collects.
4. Rinse with clean water and let the cover dry fully in fresh air before refitting it. Avoid leaving it in strong sun for hours, as this can damage the vinyl.
5. While the cover is off, check its weight and smell. A heavy, sagging or musty cover is holding water and may need replacing.

Stage 5: refill, balance and shock

1. Run the hose for a minute before filling to flush out water that has been sitting inside it. Fit a hose pre-filter if you have one.
2. Fill the tub to the level shown in your manual. Many manuals advise filling through the empty filter housing to help prevent airlocks.
3. Refit the clean filters, switch on at the isolator and let the water circulate.
4. Adjust total alkalinity first, then pH, then calcium hardness, testing between each adjustment.
5. After the last balancing chemical, let the water circulate for at least 30 minutes before you add shock.
6. Add a chlorine shock at the label dose and run the jets with the cover open, as the label directs.
7. Aim for a free chlorine level of at least 10 ppm after the shock, so that bacteria surviving the purge and scrub are killed. Use the label to find the dose that reaches this level in your water volume.
8. Run the jets for at least 30 minutes after adding shock, so the high chlorine level reaches the pipe walls.
9. Leave the cover off while free chlorine falls back to 3-5 ppm. This usually takes about 8 to 24 hours, depending on water temperature and sunlight.
10. Once the shock level has dropped, bring your normal sanitiser into the 3-5 ppm range and keep it there.

The most important tip: the first two weeks after a refill decide whether pink slime comes back. Test the water every day during that period and do not let free chlorine or bromine fall below 3 ppm. Even one day with no sanitiser gives surviving bacteria the chance to rebuild their biofilm.

What water balance stops pink slime from coming back?

Pink slime grows when sanitiser runs low, even for a short time. The targets below are widely used guidance. Some manufacturers give slightly different figures, so check your manual and follow it where it differs.

Keep a close eye on pH. As pH rises above the target range, chlorine works much less effectively. A tub can show a reasonable chlorine reading and still allow bacteria to grow if the pH has drifted too high.

At a pH above about 7.8, chlorine can lose half or more of its effectiveness. This is why bringing pH back into range comes before adding more chlorine.

Calcium hardness matters in both directions. If it is too low, the water can corrode metal parts and heater elements. If it is too high, scale forms in the pipes and on the shell, and that rough surface gives biofilm more places to grip.

Total dissolved solids (TDS) build up over time as you add chemicals and as people use the tub. Keep TDS below about 1,500 ppm. A higher reading makes your sanitiser less effective and is a sign that the water needs changing.

Ozone and UV systems reduce the work your sanitiser has to do, but they only treat water as it passes through the equipment. They leave no protection in the tub itself, so you still need chlorine or bromine at the levels above. The UK advisory body PWTAG publishes water treatment guidance for pools and spas that explains why a steady sanitiser level matters.

How do you keep pink slime away week to week?

Every few days

• Test sanitiser and pH at least two or three times a week, and after every heavy use
• Ask everyone to shower before using the tub, to reduce oils, lotions, sunscreen and sweat
• Wipe the waterline with a soft cloth
• Wipe the top of the headrests and the cup holders

Every week

• Shock the water, and shock again after a party or heavy use
• Rinse the filters with a hose
• Use plain water only on the filters. Never use soap or washing-up liquid on them
• Wipe the underside of the cover and check the seams
• When you check the underside of the cover, look for early signs of white mould as well as pink residue
• Rinse swimwear in clean water without detergent before using it in the tub

Every month and at each water change

• Test total alkalinity and calcium hardness at least once a month, not only sanitiser and pH
• Check and clean the jet faces
• Soak the filters in filter cleaner, or rotate in a clean, dry set
• Clean the cover properly, top and bottom
• Purge the pipes before every drain
• Drain and refill as often as your manual advises, which for many tubs is every three to four months

What mistakes let pink slime come back?

• Shocking the water and skipping the pipe purge
• Draining before purging, so the purge product cannot reach the pipes
• Refitting old or poorly cleaned filters into fresh water
• Cleaning the shell but forgetting the cover and headrests
• Treating only the pink slime or only the white water mould, when the two support each other
• Using an algaecide, which does not target bacterial biofilm
• Using household cleaners or washing-up liquid, which cause foam and leave residues
• Letting sanitiser drop to zero while you are away or the tub is unused
• Relying on ozone or UV alone without a measurable sanitiser level
• Letting pH drift high, which weakens chlorine
• Adding shock before balancing pH and alkalinity, because shock in high-pH water loses much of its strength
• Getting back in because the water looks clear. Clear water can still carry harmful levels of bacteria, and only a test showing sanitiser back in range confirms the tub is ready

Is pink slime in a hot tub dangerous?

For most healthy people, brief contact with pink slime is unlikely to cause serious harm. However, Serratia marcescens is a known cause of infection, mainly in people with weakened immune systems. You should not use a tub that has visible slime until you have treated it.

Serratia marcescens can enter the body through cuts, the eyes and mucous membranes. It can cause wound infections, eye infections such as conjunctivitis, and urinary tract infections.

Jets also turn tub water into a fine spray that people breathe in. If the water is contaminated, this spray can irritate the airways, which is another reason not to run the jets while bathing in an untreated tub.

The bigger concern is what the slime shows you. Biofilm that lets pink bacteria grow can also shelter other germs, such as Pseudomonas aeruginosa, which is linked to an itchy rash called hot tub folliculitis, and Legionella, which can cause Legionnaires' disease. The Health and Safety Executive guidance on Legionella explains why spa pools need careful management.

Who should take extra care

• People with weakened immune systems or long-term health conditions, including people having chemotherapy, people living with HIV and people taking immunosuppressant medicines
• People with open cuts, wounds or recent surgery
• Older people and young children, partly because young children are more likely to swallow tub water
• People with skin conditions such as eczema, or with an active skin infection
• Contact lens wearers
• Anyone who is pregnant, who should speak to their midwife before using a hot tub

When to get medical advice

Stop using the tub and get advice if you notice any of the following after bathing. Tell the health professional that you have used a hot tub, as this helps them.

• An itchy rash or red bumps, often in areas covered by swimwear
• Sore, red or sticky eyes
• Ear pain or discharge from the ear
• A cough, high temperature, muscle aches or shortness of breath
• A cut or graze that becomes red, swollen or painful

A pharmacist can advise on a mild rash. For anything more serious, or anything that does not improve, speak to your GP or use NHS 111. If someone has severe difficulty breathing, call 999. Outside the UK, contact your local health service.

When should you call a hot tub technician?

Most owners can clear pink slime themselves by following the full sequence. Some situations are better handled by a professional.

• Pink slime returns within a few weeks even after the complete purge and clean
• White water mould keeps appearing alongside the pink slime after treatment
• Free chlorine drops unusually fast despite regular dosing, which suggests a biofilm reservoir too large for a home purge product
• Heavy foam, a musty smell or cloudy water persists after refilling
• The tub has airlocks, weak jets or pumps that will not prime
• The jets or plumbing show physical damage or scale build-up
• You find a leak, or water in the equipment bay
• The heater or controls show error codes
• You are unsure which chemicals are safe to use together

If slime comes back quickly after a full treatment, the cause may be in the equipment. A cracked filter housing can let water bypass the filter, and a faulty ozonator, UV unit or sanitiser feeder can leave the water less protected than you expect. A technician can check these parts.

A damaged plumbing seal or joint is another possible cause. It can let contaminated water from outside the system get in, such as groundwater around an in-ground tub, and that water brings fresh bacteria with it. Ask a technician to check the seals and joints if the slime keeps returning.

A technician can also check for sections of pipe where water never flows, which can hold biofilm that no purge reaches. Leave any work on the electrical side of the tub to a qualified person, and always switch off at the isolator before anyone opens the equipment bay.

Avoid repeating the full DIY treatment again and again on a tub with an underlying fault. Each round only delays finding the real cause and adds cost. After one full treatment has failed, a technician's inspection is usually cheaper than more rounds of chemicals and another drain and refill.

Pink slime: the short version

Pink slime is bacteria living in a protective biofilm, not algae. Shock kills what floats in the water but leaves the colonies in the pipes, filters and cover, which is why the slime keeps coming back.

Purge the pipes in warm water before you drain. Then clean the shell, the filters and the underside of the cover, refill with a flushed hose, balance the water and shock. After that, test daily for two weeks and keep sanitiser at 3-5 ppm with pH at 7.2-7.6. Steady sanitiser is what keeps pink slime away for good.

Frequently asked questions

Is the pink slime in my shower the same as in my hot tub?

Often, yes. Serratia marcescens is a common cause of the pink film on shower grout, sink drains and toilet bowls. It lives in damp places around the home and can reach a hot tub on people, towels or swimwear. This is one reason why showering before you bathe and keeping sanitiser steady matter so much.

How soon can I use the hot tub after treating pink slime?

Wait until the shock has dropped and your sanitiser reads within 3-5 ppm, with pH between 7.2 and 7.6. Check the shock label for any minimum waiting time, and test the water yourself rather than guessing. Also make sure the water has heated to a comfortable temperature, no hotter than 40°C, before anyone gets in.

How often should I purge my hot tub pipes?

Purge the pipes before every drain and refill. For many tubs that is every three to four months, but your manual may give a different interval based on tub size and how often it is used. If pink slime has been a problem, purging at every water change is the single habit that makes the biggest difference.

Will an ozonator or UV system stop pink slime?

These systems help, because they kill some bacteria and break down waste as water passes through them. They do not leave any protection in the tub itself or inside pipes where water sits still. You still need a measurable level of chlorine or bromine at all times. Treat ozone and UV as support for your sanitiser, never as a replacement.

Do I need to replace my filters after pink slime?

Not always. A thorough soak in filter cleaner followed by full drying clears many cartridges. Replace a filter if the pleats are damaged, the end caps are cracked, it is near the end of its expected life, or the pink colour returns soon after cleaning. A badly contaminated old filter can reseed fresh water quickly.

Can pink slime grow in a bromine hot tub?

Yes. Pink slime can grow in bromine tubs just as it does in chlorine tubs, especially if bromine levels drop or pH drifts high. The removal sequence is the same. Many owners use a chlorine shock in a bromine tub, but check your manual first, then keep bromine at 3-5 ppm after the refill.

How do you treat pink slime in a swimming pool?

Brush every affected surface first to break up the biofilm. Then shock the pool to at least 10 ppm free chlorine and hold that level for about 24 hours. Afterwards, keep free chlorine above 3 ppm so the bacteria cannot settle again. A pool does not have the hidden jet plumbing of a hot tub, so the pipe purge is a hot tub step only.