My Hot Tub Filter Clogged After Two Uses — What on Earth Was in It?

I recently bought a hot tub.

It was not something I had particularly planned to buy. While I was away on holiday, I tried one properly for the first time and discovered just how relaxing sitting in warm, bubbling water could be. It was one of those experiences that makes you think, I could get used to this.

So, when I returned home, we decided to invest in one.

The hot tub arrived, we set it up, filled it, got the water chemistry sorted out and eventually reached that wonderful moment when the water was warm enough to climb in.

Everything worked beautifully.

Then, after only two days and two sessions of about 30 minutes each, the hot tub stopped and displayed an error code.

The problem appeared to be restricted water flow.

I removed the filter.

It was filthy.

That genuinely surprised me.

I had showered before getting into the hot tub. I certainly didn't feel dirty. The water itself still looked reasonably clean. Yet the filter told a very different story.

Surely I wasn't supposed to throw away a filter every couple of days?

So I started investigating.

And it turns out that owning a hot tub provides a surprisingly good lesson in filtration, chemistry, microbiology — and just how much material apparently comes off perfectly clean-looking human beings.

The Filter Was Actually Doing Its Job

My first reaction was that something had gone wrong.

In reality, discovering material trapped in the cartridge was evidence that at least one part of the system was working exactly as intended.

A hot-tub filter is normally made from pleated synthetic material. The large number of pleats creates a considerable surface area through which water can pass while particulate contamination becomes trapped.

BISHTA — the British and Irish Spa and Hot Tub Association — describes the filter as a critical part of the system because it removes solid and particulate material from the circulating water.

There is, however, an important distinction.

Filtration does not equal disinfection.

The filter physically removes material.

The sanitiser — normally chlorine or bromine — deals with microorganisms.

Water chemistry controls factors such as pH and hardness.

Circulation moves the water through the system.

They all have to work together.

That was probably my first lesson: I had subconsciously thought of the filter rather like the filter in a vacuum cleaner. Put a clean one in, wait until it is dirty and then replace it.

A spa cartridge is rather different.

It is designed to be regularly cleaned and reused, subject to the manufacturer's instructions.

So a dirty filter does not necessarily mean:

"I need another filter."

It often means:

"I need to clean this filter."

But I'd Had a Shower — So Where Did the Dirt Come From?

This was the part that intrigued me.

I had deliberately showered before entering the hot tub. Surely that should have removed most of the contamination?

It certainly helps.

Guidance from the Pool & Hot Tub Alliance recommends showering immediately before entering because doing so removes things including perspiration, body oils and suntan lotions.

But showering doesn't turn us into sterile objects.

Far from it.

1. Natural skin oils

Our skin constantly produces sebum — an oily mixture that helps protect it.

You may have washed shortly before getting into the hot tub, but your skin still contains oils.

Put somebody into warm water and some of those substances inevitably enter the water.

BISHTA specifically warns that greases and fats can become ingrained in the pores of filter material, reducing water flow.

That description sounded remarkably similar to what I had apparently experienced.

2. Perspiration

You don't have to feel sweaty to introduce perspiration into the water.

And there is something slightly ironic about a hot tub.

We sit in hot water to relax.

The heat warms us.

Our bodies respond to heat partly by sweating.

Some of what the water-treatment system has to handle is therefore being introduced by the people sitting in it.

3. Dead skin cells

Human skin is continually renewing itself.

Tiny fragments of skin are being shed all the time, whether we notice them or not.

Most are microscopic.

Put people into a relatively small volume of warm, circulating water and some of this material eventually reaches the filtration system.

The filter then concentrates it.

What was invisible when dispersed through perhaps hundreds or thousands of litres of water suddenly becomes rather more obvious when collected on a small cartridge.

Perhaps the Biggest Surprise: My Swimming Costume

It may not all have been coming from me.

Some could have been coming from what I was wearing.

We wash swimming costumes.

And what do we wash them with?

Detergent.

Possibly fabric conditioner as well.

Even after rinsing, tiny quantities can remain trapped within the fibres.

When the costume is subsequently immersed in warm water, some can be released.

Jacuzzi notes that residual detergents from swimwear can contribute to spa problems and recommends thoroughly rinsing swimwear and avoiding fabric softener.

That is something I had never really considered.

My perfectly clean swimming costume could actually introduce substances that my hot tub would rather not have.

Then There Are All the Things We Put Onto Ourselves

Think about what might be on somebody before they get into a hot tub:

moisturiser;
sun cream;
deodorant;
makeup;
hair conditioner;
hair styling products;
perfume;
aftershave;
body lotion;
soap residue;
shampoo;
laundry detergent from swimwear.

Manufacturers identify many of these as common contributors to foaming and contamination in spa water.

Individually, the quantities may be tiny.

But a hot tub contains a relatively small volume of water compared with a swimming pool.

Imagine putting one teaspoon of something into a large swimming pool.

Now put that same teaspoon into a hot tub.

Its concentration is vastly greater.

Add two, three or four people, and the bather load starts to become significant surprisingly quickly.

The Filter Is a Concentrator

This helped me understand why the cartridge looked so dirty.

The filter isn't creating dirt.

It is collecting it.

Imagine passing hundreds or thousands of litres of apparently clear water through the same piece of filtration material over and over again.

Every circuit removes another small amount.

Eventually all those individually microscopic quantities become a visible deposit.

In a sense, the filter is giving us a much more truthful picture of the water than our eyes can.

Clear water is not necessarily clean water.

And dirty-looking filter material doesn't automatically mean the water was horrendously dirty.

It means the filtration system has concentrated contaminants into one place.

Why Did It Cause an Error Code?

This is where the problem changes from being cosmetic to mechanical.

A filter requires water to pass through it.

As contamination accumulates between and inside the pleats, resistance to flow increases.

Eventually the pump has to work harder to move the same amount of water.

BISHTA warns that contamination embedded in the filter can reduce flow and put excessive stress on the circulation pump.

Some hot tubs monitor water flow.

If the control system concludes that circulation is insufficient, it may shut down heating or other systems and report a low-flow or filtration error.

That is actually sensible.

A heater operating without sufficient water flowing through it could potentially be damaged.

A clogged filter can therefore produce a symptom that initially looks like an electrical or mechanical failure.

Jacuzzi's troubleshooting guidance similarly lists a dirty filter among the things to investigate when a spa reports a low-flow condition, alongside matters such as air locks and circulation problems.

Obviously, individual error codes vary between manufacturers, so the correct response is always to check the manual for the particular model rather than assume every flow error means "dirty filter".

But in my case, finding a badly contaminated filter provided a very good clue.

So Do I Really Need a New Filter Every Two Days?

Fortunately, no — not normally.

That was perhaps the most useful discovery of my research.

Filters are designed to be cleaned.

For general hot-tub cartridge filters, BISHTA recommends a routine that includes rinsing them weekly and carrying out a more thorough specialist clean approximately monthly, while always following the spa and filter manufacturer's own instructions.

My experience suggests that, at least while the hot tub is new and I am learning how quickly my particular system loads its filter, I will inspect it considerably more often.

If it looks dirty, I'll clean it.

If circulation begins to fall, I'll investigate it.

There is little point stubbornly waiting until "filter cleaning day" if the filter is already telling me that it needs attention.

Cleaning Is Not the Same as Blasting It

There is another trap here.

A dirty filter looks like something that would benefit from a pressure washer.

It would certainly become clean quickly.

Unfortunately, it might also become useless.

BISHTA specifically advises not to use a pressure washer, because the powerful jet can damage the filter fibres.

The basic method is much gentler:

Switch off the hot tub as instructed by the manufacturer.
Remove the cartridge.
Rinse between the pleats with a garden hose.
Rotate the cartridge so that the complete surface is cleaned.
For periodic deeper cleaning, use a specialist spa-filter cleaning product according to its instructions.
Rinse it extremely thoroughly afterwards.
Allow it to dry when appropriate before returning it to service.

Ordinary washing-up liquid is definitely not the answer.

Adding detergent to something whose job is to remove detergent would be rather self-defeating.

BISHTA specifically advises against household detergents and cleaning products for cartridge cleaning.

Two Filters May Be Better Than One

One particularly practical idea I discovered is simply to own a spare.

Rather than removing the cartridge, cleaning it and wanting to use the hot tub immediately afterwards, I can alternate between two.

Filter A comes out.

Filter B goes in.

Filter A can then be cleaned properly, rinsed and allowed to dry.

BISHTA actually recommends a two-set rotation for suitable filters for exactly this reason.

That makes far more sense to me than keeping a pile of disposable replacements and throwing one away every time it looks dirty.

For smaller inflatable spas and proprietary cartridges, recommended replacement intervals may be quite different, so the manufacturer's guidance still takes precedence.

What I Am Going to Change Before Getting Into the Hot Tub

My first instinct might have been to conclude:

I need better filters.

I now think the better starting point is:

I need to reduce what reaches the filters in the first place.

My pre-hot-tub routine will therefore become a little more deliberate.

Shower immediately beforehand

A quick rinse immediately before getting in reduces perspiration, body oils and cosmetic products reaching the water.

If I have used sun cream, moisturiser or similar products earlier in the day, a more thorough shower may make sense.

Keep a swimming costume specifically for the hot tub

Rather than repeatedly introducing laundry products into the water, a dedicated costume can be thoroughly rinsed.

I will also avoid fabric conditioner on it.

Keep feet clean

Walking across a patio, lawn or garden and then stepping directly into the hot tub is an excellent way of turning the filter into a miniature soil collector.

A small footbath or quick rinse before entering may save considerable filtration work.

Keep the cover on

Leaves, insects, pollen, dust and other airborne debris do not need invitations.

If nobody is using the hot tub, the cover does a lot more than retain heat.

It also reduces contamination.

The Filter Cannot Do Everything

This investigation also reminded me of something much more important.

A physically clean hot tub is not necessarily a hygienically safe hot tub.

A filter can remove particles.

It cannot be relied upon to kill microorganisms.

For that, proper sanitisation is essential.

BISHTA recommends maintaining a residual sanitiser such as chlorine or bromine, rather than relying on filtration or supplementary systems alone.

The CDC similarly recommends checking sanitiser and pH before using hot tubs; its current guidance specifies at least 3 ppm chlorine or 4–8 ppm bromine and pH between 7.0 and 7.8. Your own spa manufacturer's treatment instructions should also be followed.

This isn't chemistry for chemistry's sake.

Warm water is potentially a very comfortable environment for microorganisms too.

The objective is therefore not simply:

Does the water look clear?

It is:

Is the water properly filtered, circulated, balanced and sanitised?

Hard Water Adds Another Complication

Living in Britain presents another variable: the water coming from the tap.

BISHTA notes that hard water is particularly common in southern and eastern parts of the UK. Excessive calcium hardness can contribute to cloudy water and scale deposits, including scale within spa equipment.

Scale is very different chemically from body oil, but from the filter's perspective it is still something that can interfere with efficient operation.

So if filters repeatedly become difficult to clean, or deposits appear white or crusty rather than greasy, investigating water hardness may be worthwhile.

This is where a decent water-testing routine becomes much more valuable than simply adding chemicals whenever the water "doesn't look right".

A Second Surprise From My Research: Thirty Minutes May Be Too Long

My filter wasn't the only thing I learned about.

I mentioned that we had enjoyed two sessions of approximately 30 minutes.

While researching hot-tub maintenance, I discovered that BISHTA recommends a maximum bathing period of 15 minutes before taking a break, together with keeping properly hydrated. It also advises that hot-tub water should never exceed 40°C.

That doesn't mean I have to abandon the relaxing evening hot tub.

It means I can have 15 minutes, get out, cool down, drink some water and then decide whether I want another session.

It was a useful reminder that when buying a new piece of equipment, reading about how to use it safely is just as important as learning which buttons to press.

My New Hot-Tub Routine

After this first little adventure, my routine will probably look something like this:

Before getting in

shower;
minimise lotions and cosmetic products;
use well-rinsed swimwear;
rinse feet;
test the water;
check that circulation looks normal.

After use

check the water;
treat it according to the manufacturer's water-care programme;
replace the cover.

Regularly

inspect the filter;
rinse it when required;
carry out proper periodic filter cleaning;
clean around the waterline;
monitor pH and sanitiser;
inspect for scale, foam or unusual deposits.

And importantly

keep a spare cartridge available.

That final point alone should make owning the hot tub considerably less frustrating.

What Does the Dirty Filter Actually Tell Me?

Perhaps this is my favourite part of the whole experience.

When I first pulled out that filter, I saw a problem.

Now I see information.

A greasy deposit might tell me that too much body oil, lotion or detergent is entering the water.

A crusty deposit might point towards scale.

A filter that blocks extraordinarily quickly might suggest heavy contamination, inadequate pre-rinsing, a water-balance problem or another circulation issue that needs investigation.

Foaming can suggest detergents, oils or cosmetic products.

Cloudiness may indicate filtration or water-chemistry problems.

The filter has become something of a diagnostic tool.

Instead of simply throwing it away, I can ask:

What is this filter telling me about what is happening in the water?

A Hot Tub Is Really a Small Water-Treatment Plant

Perhaps that is the biggest change in my thinking.

I bought a hot tub because I wanted somewhere warm and relaxing to sit.

What I actually bought was a small water-treatment system.

There is a pump.

There is filtration.

There is heating.

There is chemical treatment.

There is water testing.

There is microbiology.

There is flow control.

There is even some rather interesting fluid dynamics as air and water are pumped through the jets.

And there is a human input to the whole system — us.

We bring oils, perspiration, microscopic skin particles, detergent residues and assorted cosmetic products with us.

The filtration and treatment systems then have to deal with them.

Conclusion: The Dirty Filter Was a Useful Lesson

After only two uses, I was initially rather horrified to discover how dirty my hot-tub filter had become.

I had showered.

I thought I was clean.

And yet the cartridge clearly disagreed.

But the more I investigated, the more interesting the problem became.

The answer isn't that we are walking around covered in huge quantities of visible dirt. It is that very small quantities of many different substances become significant when several hundred litres of water are repeatedly circulated through the same fine filter.

Skin oils, perspiration, microscopic skin particles, cosmetic products, sunscreen, shampoo residues and detergent carried in swimwear can all contribute.

The solution isn't to buy a new filter every couple of days.

It is to understand the system.

Reduce contamination before entering.

Clean the filter properly.

Keep a spare.

Maintain water chemistry.

Keep the sanitiser correct.

And pay attention to what the filter is telling you.

My hot tub was purchased for relaxation.

Unexpectedly, it has also become another science experiment.

And I suspect this will not be the last lesson it teaches me.

Sunday, 16 August 2026