Hot tub mineral sanitisers and what they really do

A hot tub mineral sanitiser is a cartridge that slowly releases silver, and sometimes copper, into your water. These metals help hold back bacteria and algae, so you can usually run lower chlorine or bromine levels. They do not replace chlorine or bromine. You still need a small, measurable residual of one of them every day.

This guide explains how minerals work, what they can and cannot do, and how to choose and fit a cartridge. It also covers which water care systems they work with, and how to stop copper from staining your shell or your hair.

What is a hot tub mineral sanitiser?

Most mineral sanitisers are small cartridges filled with a silver compound, sometimes mixed with copper or zinc. Water flows through or around the cartridge and picks up a very low, steady level of metal ions. The cartridge either floats in the tub, slots into the core of your filter, or sits in a dedicated housing built into the tub.

Some hot tub manufacturers fit their own silver-based purifier as standard or as an option. Others leave it to the owner to add an aftermarket cartridge. Either way, the principle is the same: a slow-release mineral working alongside a primary sanitiser.

How silver and copper act on germs

Silver ions attach to bacteria and damage their cell walls and the enzymes they need to survive. This works, but it works slowly, far more slowly than chlorine or bromine. That is why minerals are described as a supporting sanitiser rather than a main one.

Silver acts against bacteria at very low concentrations. Laboratory research has reported an effect from around 0.006 mg/L, which is a tiny amount. That is why a cartridge only needs to release a trace of silver to be useful, and why it can last for months.

Copper ions mainly hold back algae. Algae is less common in a covered, hot tub than in an outdoor pool, but copper still helps keep surfaces and filters cleaner. Neither metal breaks down the sweat, body oils, cosmetics and lotions that bathers leave behind. That job belongs to chlorine, bromine or a non-chlorine shock.

Copper ions also act against some bacteria that silver alone may not fully control. Because copper adds this to its effect on algae, the two metals complement each other. This is why many cartridges combine silver and copper rather than using one metal on its own.

Why minerals help most between doses

At normal hot tub temperatures of 38-40°C (100-104°F), chlorine is used up quickly by heat, sunlight and bathers. Its level can drop noticeably within hours of dosing. Silver ions are not used up in the same way, so they keep giving low-level protection at those temperatures.

This is where minerals earn their place. They act as a background safety layer while your chlorine or bromine level dips between doses, which helps keep the water steadier from one day to the next.

Why spa minerals differ from pool minerals

Pool mineral products are designed for tens of thousands of litres of cooler water. A typical hot tub holds around 1,500 litres (about 400 US gallons) of water at up to 40°C (104°F), with far more bathers per litre. A pool product in a hot tub can release far too much metal for that small volume.

Too much metal, especially copper, is the main cause of staining and green-tinted hair. Only use a cartridge that the maker states is made for hot tubs or spas, and that is sized for your water volume.

A pool-strength mineral stick in a hot tub can give the water a green tinge within about two weeks. Owners often mistake this for algae and add more sanitiser, when the real cause is too much copper. If the water turns green soon after fitting a pool product, remove it and test for copper before doing anything else.

Can minerals replace chlorine or bromine?

No. Minerals reduce how much chlorine or bromine your water uses, but they cannot do the whole job. Hot tubs are warm, heavily used and full of organic matter, which is the perfect setting for bacteria such as Legionella and Pseudomonas. You need a sanitiser that kills quickly and leaves a residual you can test.

How big is the reduction? Mineral cartridge makers often claim you can cut the chlorine or bromine you need by up to about half. Treat that as a rough upper figure from manufacturers, not a promise. Your own saving depends on how often the tub is used and how well the water is balanced.

Many mineral cartridge makers say their product lets you run free chlorine at around 0.5-1 ppm. Without minerals, the widely accepted range for hot tubs is 3-5 ppm free chlorine or 3-5 ppm bromine. Follow the figure in your cartridge instructions and your hot tub manual, and use the higher end after heavy use.

Makers' figures do vary. Some give 1-2 ppm free chlorine rather than 0.5-1 ppm, and with bromine a level of around 2-3 ppm is typical instead of the usual 3-5 ppm. Always use the figure printed on your own cartridge, not a number from a different product.

Bromine is generally more stable than chlorine at hot tub temperatures, and it has less smell. Paired with minerals, it tends to give the softest-feeling water of the common options. If gentle water is your main reason for choosing minerals, bromine is worth considering as the partner sanitiser.

If you use chlorine, the usual choice for hot tubs is stabilised granular chlorine, known as sodium dichlor. It dissolves quickly and is easy to dose in small amounts. Other forms, such as slow-dissolving trichlor tablets or unstabilised liquid chlorine, are generally less suited to hot tubs, so check your manual before using them.

UK guidance for commercial spa pools, from the HSE and PWTAG, is based on chlorine or bromine disinfection. A home tub is not a commercial pool, but the lesson is the same: a lower residual leaves you less margin for error, so you need to test more carefully, not less.

Never run a hot tub on minerals alone. Always keep a measurable level of chlorine or bromine in the water. If you or anyone who has used the tub develops a rash, sore eyes, or a cough, fever or flu-like symptoms in the days after, stop using it and speak to a pharmacist or GP. The NHS has more on Legionnaires' disease.

What are the real benefits and limits?

What minerals do well

• Lower chlorine or bromine levels, which many people find gentler on skin and eyes and less strong in smell.
• Steadier water between doses, because the minerals keep working while the sanitiser level dips.
• Softer-feeling water, which many owners report, although this is a personal impression rather than a measured effect.
• Less algae and slime on surfaces and in the filter.
• Low effort: you fit a cartridge and change it every few months.

Where minerals fall short

• They do not oxidise organic waste, so you still need to shock regularly.
• They act slowly, so they cannot respond to a sudden heavy bather load.
• Copper can stain shells and tint light hair if pH drifts or the dose is too high.
• They are not compatible with every water care system, especially biguanide.
• A lower sanitiser residual means less margin, so careless testing becomes more risky.
• They cut only your chlorine or bromine spend, not your spend on pH adjusters or shock.

Who minerals suit, and who they do not

Minerals tend to suit the following owners best. If anyone in your household has a skin condition that flares after bathing, speak to a GP or pharmacist, as a lower chlorine level is not a treatment.

• People with sensitive skin or eczema who find standard chlorine or bromine levels irritating.
• People who dislike the smell of chlorine or are bothered by the fumes above the water.
• Families whose children use the tub often.
• Busy tubs, used daily or by several people, where chemical use is high.

Minerals do not suit anyone who wants a single product with no chemical management. You still need to test, balance, dose and shock. If that sounds like too much, a mineral cartridge will not remove the work, it only makes it a little lighter.

What do they cost to run?

Replacement cartridges typically cost somewhere between about £20 and £70, depending on the type and whether it is a manufacturer's own part. Most last a few months, so you are likely to buy three or four a year. Check current prices, as they vary widely between retailers.

Against that, you will usually use less chlorine or bromine. Whether you save money overall depends on how much you use the tub and what you currently spend on chemicals. Choose minerals for steadier, gentler water, not as a guaranteed saving.

Keep in mind that only the chlorine or bromine part of your spending goes down. You still buy the same pH adjusters and the same shock. Once replacement cartridges are added in, the yearly running cost is often about the same as a chemical-only routine, or slightly higher.

How do you choose a hot tub mineral sanitiser?

Start with what your hot tub manual says. Some manufacturers approve specific mineral systems or include their own, and using an unapproved product may affect your warranty. If the manual is silent, compare cartridges on the features below.

• Format: a floating dispenser is the easiest to fit, a filter core insert stays out of sight, and a built-in housing is tidy but usually needs a matching part.
• Sizing: the cartridge must be rated for your water volume, ideally with an adjustable dial if your tub is at the edge of its range.
• Partner sanitiser: check whether it is made to work with chlorine, bromine, or both.
• Metals used: silver-only cartridges carry less staining risk than those with copper.
• Lifespan: look for a clear replacement interval or a visible indicator.
• Supply: make sure replacements will be easy to get in your country.

If your tub has a brand-specific filter housing and nobody makes a mineral cartridge to fit it, use a floating dispenser instead. A floater does not depend on the housing at all, so it works in almost any tub, as long as it is rated for your water volume.

Most hot tub mineral cartridges are rated for roughly 750 to 2,300 litres (about 200 to 600 US gallons). Some budget cartridges stop at around 1,900 litres (500 US gallons). Find your tub's volume in the manual and check it against the cartridge rating before you buy.

A silver-only cartridge gives no algae control, because algae control comes from copper. With silver alone, algae prevention depends fully on your chlorine or bromine, so keep that level steady and never let it fall to zero.

How your pump schedule affects a cartridge

In tubs where the pump runs only part of the day, metal ions tend to collect near the filter while the pump is off. Tubs with 24-hour circulation spread them more evenly through the water. If your tub uses filtration cycles, a longer daily cycle helps the cartridge work better.

Combined mineral and sanitiser floaters

Some floating units combine a mineral cartridge with a slow-release chlorine or bromine cartridge that is designed to regulate its own output. The sanitiser part is usually replaced every three to four weeks, while the mineral part lasts about four months.

These suit tubs that go several days between checks, such as holiday lets or second homes. You still need to test and shock, but the water is less likely to run out of sanitiser between visits.

Combined floaters cost more to run than a standalone mineral cartridge. The reason is the chlorine or bromine part, which you must replace every three to four weeks on top of the mineral part. Add both refills into your yearly figure when you compare the two options.

Any floating dispenser can drift against a jet or a suction fitting and get caught or lodged there. That can block water flow or damage the dispenser. Check where it is floating each time you open the cover, and move it if it has settled near a fitting.

Price bands

As a rough guide to price bands, generic floating cartridges sit at the lower end. Filter core inserts are usually mid-range. Systems designed for one brand of hot tub tend to cost the most, and may be your only option if your tub has a dedicated housing.

When you compare prices, work out the cost per month of life: divide the price by the number of months the cartridge lasts. A cheaper cartridge that lasts three months can cost as much as, or more than, a dearer one that lasts four. The shelf price alone does not tell you which is better value.

If you are new to minerals, a lower-cost generic cartridge is a sensible way to try the approach first. Use it for one full cartridge life and see whether you like the water and the routine. Then decide whether a longer-lasting or premium cartridge is worth the extra cost.

Mistakes to avoid

• Buying a pool mineral product for a hot tub.
• Using two mineral products at once, which doubles the metal in the water.
• Adding minerals to a biguanide system.
• Stopping regular testing because the water looks clear.
• Keeping a cartridge past its replacement date to save money.
• Buying a cartridge rated for less water than your tub holds.

Will a mineral cartridge work with your hot tub?

Run through this checklist before you buy. If you answer no or are unsure on any point, contact the cartridge maker or your hot tub dealer first.

• Your main sanitiser is chlorine or bromine, and the cartridge is approved for it.
• You do not use biguanide (PHMB) or a biguanide-based system, which is generally not compatible with minerals.
• Your filter or housing fits the cartridge size and shape.
• If you have a salt system, its maker approves adding minerals.
• Your fill water is not already high in copper or iron, which is more common with private wells and boreholes.
• Your shock product is compatible: many mineral systems recommend non-chlorine shock (MPS), and most also accept chlorine shock.
• You have checked with the cartridge maker before using a metal sequestrant, as it may also bind the minerals you want in the water.

Ozone and UV systems generally work alongside minerals without any problem, as they treat the water in a different way. If you are still deciding on your main sanitiser, read our guide to bromine vs chlorine for your hot tub first.

How do you fit and use a mineral cartridge?

Fitting takes a few minutes. Your cartridge instructions always come first, but most follow the same pattern.

Before you start, gather everything you need: test strips or a digital tester, your chlorine granules or bromine tablets, and your shock product. With these to hand, you can finish the whole setup in one go, usually in under 15 minutes.

Where convenient, fit your first cartridge straight after a fresh drain and refill. Old water may carry biofilm or a chemical imbalance, and either can undermine the cartridge from the start.

Get the pH right before the cartridge goes in. If pH is outside 7.2-7.6 at installation, the ions work less effectively and the risk of copper staining starts from day one. Correct the pH first, then fit the cartridge.

1. Switch the hot tub off at the isolator.
2. If you are replacing an old cartridge, remove it and check the filter core or housing for scale or discolouration. Either is a sign of a water balance problem, so fix the balance before fitting the new one.
3. Test and balance the water: pH 7.2-7.6, total alkalinity 80-120 ppm and calcium hardness 150-250 ppm.
4. If the cartridge has a dial, set it to your tub's water volume.
5. Fit the cartridge: place it in the filter core or housing, or put the floating type in the water.
6. Refit the filter, close the lid of the housing, and switch the power back on.
7. Run the jets for about 15 minutes so the ions spread through the whole tub before you test.
8. Bring chlorine or bromine to the level given in your cartridge instructions.
9. After about 30 minutes, once the jets have circulated the water, test pH, alkalinity and sanitiser again.
10. Write the fitting date directly on the cartridge with a permanent marker, and also set a reminder on your calendar or phone.
11. Test again after 24 hours and adjust the sanitiser dose if needed.

Lower your sanitiser target on purpose

Fitting the cartridge is only half the change. You must also lower your chlorine or bromine target to the figure your cartridge maker gives. If you keep dosing exactly as before, you get none of the benefit of gentler water or lower chemical use.

The opposite mistake is just as common. Some owners fit a cartridge and stop shocking, thinking the minerals now cover that job. They do not, and the water soon turns dull, smelly or cloudy. Lower the daily sanitiser level, but keep the weekly shock.

How often should you test?

• Chlorine or bromine and pH: before each use, and at least three times a week.
• Total alkalinity: once a week.
• Calcium hardness: once a month and after each refill.
• Total dissolved solids (TDS): once a month, at the same time as calcium hardness.
• Copper: once a month if your cartridge contains copper, and straight away if you see stains or green-tinted water.

Because your sanitiser level is lower with minerals, it can fall to zero faster after a busy evening. Testing before you get in is the simplest way to catch that. Our guide to the hot tub chemicals you actually need covers test kits and the basic chemical set.

When should you shock?

Shock once a week, and after any heavy use such as a party or several people bathing at once. Shock oxidises the organic waste that minerals cannot touch. It also helps stop slime building up in the pipes.

As a simple rule, treat any session with four or more bathers as heavy use, and shock the water afterwards.

Shock straight away, without waiting for the weekly dose, if the water turns cloudy or develops a smell. Both are signs that organic waste is building up faster than your low sanitiser level can deal with.

Many mineral systems recommend a non-chlorine shock based on potassium monopersulphate (MPS). Chlorine shock is also accepted by most, but check your cartridge instructions. Add shock with the cover off and the jets running, and wait for levels to return to normal before bathing.

The two types behave differently. Non-chlorine shock oxidises waste while leaving your chlorine at the lower mineral-system level. Chlorine shock is stronger, but it pushes free chlorine well above that range. After a chlorine shock, wait roughly 12-24 hours, or until a test shows the level has fallen back, before anyone bathes.

When should you replace the cartridge?

Replace it when its stated lifespan ends, which is often every three to four months but varies between products. Many makers also suggest fitting a new cartridge each time you drain and refill. If your cartridge has an indicator, trust that over the calendar.

A depleted cartridge releases no useful ions, but it still sits in the filter or floats in the water and can look exactly as it did on day one. That can give you false confidence that the water is protected. Replace it on schedule even if it looks intact.

When you change the cartridge at a drain and refill, test every parameter both before and after the water change. The first test shows how the old water was performing. The second confirms that your baseline has reset in the fresh water before you fit the new cartridge and set your sanitiser level.

Can copper stain your hot tub?

Yes, it can. Copper stays dissolved when water is balanced. If the pH rises too high, it can come out of the water and settle on the shell. If the pH falls too low, the water can also pull extra copper from metal parts, which then stains later.

The risk rises sharply once pH goes above about 7.8. At that point copper is most likely to come out of the water and stain the shell or tint hair. This is one more reason to correct a high pH reading promptly rather than leaving it for a few days.

Without minerals, a pH of up to 7.8 is often treated as acceptable in a hot tub. With a cartridge that contains copper, that margin is too wide. Hold pH within the tighter range of 7.2-7.6 so it stays well below the point where staining becomes likely.

Signs of too much copper

• Blue-green, grey or dark marks at the waterline or around jets.
• Clear water that looks faintly green or teal.
• A green tint in blonde, grey or chemically treated hair.
• A copper test reading above the figure your cartridge maker gives.

Most standard hot tub test strips do not measure copper. To do the monthly check, you need a separate copper or metals test strip or kit. Keep it with your usual strips so the check becomes part of your routine.

Green water can also be algae, so check your sanitiser level too. Our guide on why hot tub water turns green explains how to tell metal from algae.

How to prevent staining

1. Keep pH between 7.2 and 7.6, and test it at least three times a week.
2. Keep total alkalinity between 80 and 120 ppm, so the pH stays steady.
3. Choose a silver-only cartridge if you have light hair in the family or a pale shell.
4. Use only one mineral product, sized for your water volume.
5. Test your fill water for metals if it comes from a well or borehole.
6. If a test shows high copper, remove the cartridge and ask its maker how to bring the level down.

What to do if copper staining has already happened

1. Remove the mineral cartridge so it stops adding copper.
2. Lower pH temporarily to about 7.0-7.2.
3. Add a spa metal sequestrant, following the dose on the label. It binds the dissolved copper so the filter can remove it.
4. Avoid heavy chlorine shock during this time, as it can push copper back out of the water.
5. Run the jets and clean the filter as the sequestrant works.
6. Return pH to the normal range of 7.2-7.6.
7. Before refitting, check that the cartridge is rated for your water volume, and set any dial correctly.

The most important habit with any mineral system is keeping pH between 7.2 and 7.6. Balanced pH protects your shell from copper stains and helps your chlorine or bromine work properly at its lower level.

How do minerals compare with ozone and salt?

Ozone, salt and minerals are often grouped together, but they do different jobs. An ozonator injects ozone gas into the water, which oxidises waste and kills germs quickly, but it leaves almost no residual. You can read more in our guide to what a hot tub ozonator does.

Ozone is made in one of two ways: by a UV lamp, or by a corona discharge unit that uses an electrical charge. Either way, ozone breaks down very quickly and has gone from the water within seconds of injection. That is why it cannot replace a sanitiser you can test.

A salt system is not a chlorine-free option: it makes chlorine from dissolved salt. Minerals add slow-acting metal ions. All three reduce how much chemical you add by hand, and all three still rely on chlorine or bromine to keep the water safe.

Because a salt system produces chlorine steadily and continuously, the level stays more even than with hand dosing. Many owners find this less irritating than chlorine granules added by hand, which cause short peaks after each dose.

Cost is a real difference. A salt system costs far more to install up front than a mineral cartridge, often hundreds of pounds or more. A mineral cartridge is a small, repeat purchase instead. If budget matters, that gap is worth weighing before you choose.

Minerals and ozone are a common and compatible pairing. Adding minerals to a salt system is possible with some equipment but not all, so check with the maker. Our article comparing saltwater and chlorine hot tubs covers the cost and care of salt in more detail.

Running ozone together with a mineral cartridge and low-dose chlorine is often seen as the most complete low-chemical setup. Ozone handles quick oxidation, minerals give steady background protection, and chlorine provides the testable residual that keeps the water safe.

What about biofilm and cloudy water?

Biofilm is a slimy layer of bacteria that grows inside pipes and jets, where it is protected from sanitiser. Minerals may slow its growth, but they will not remove biofilm once it has formed. Signs include a slippery feel on surfaces, persistent foam, cloudy water, flakes from the jets, and chlorine or bromine that disappears unusually fast.

In its early stages, biofilm is invisible. It grows mainly on pipe walls, jet fittings and filter housings, where water flow is intermittent and sanitiser reaches it less often. Because of this, it is usually well established before owners notice any of the signs above.

Biofilm is a health concern, not only a cosmetic problem. It can shelter harmful bacteria such as Legionella and Pseudomonas, protecting them from the sanitiser in the water. That is why it needs treating promptly rather than ignoring while the water still looks acceptable.

Two more signs are worth watching for:

• Cloudiness that does not clear after you have shocked and balanced the water.
• A foam ring that comes back soon after you skim it away.

If you see two or more of these signs, treat for biofilm before anyone bathes again. Do not simply add more chlorine or bromine, because extra sanitiser alone will not clear biofilm that is protected inside the pipes.

Match the treatment to how bad it is

• Foam ring only: a pipe cleaner and a shock usually clear it in about 4 to 6 hours.
• Cloudy water with slimy surfaces: pipe cleaner and shock, plus a thorough filter clean, taking about 8 to 12 hours.
• Biofilm that persists after shocking: a full drain, clean and refill, taking about 24 hours.

Treating a mild case without draining

A strong shock and a filter clean can improve things for a short time without draining. For a mild case, a more thorough treatment without a drain usually follows these steps.

1. Remove the mineral cartridge.
2. Add a hot tub pipe cleaner at the dose on the label, then run all jets on full for about 30 minutes.
3. Raise free chlorine to about 10-20 ppm and hold it there for 2-4 hours with the jets running.
4. Clean the filter thoroughly.
5. Keep everyone out of the tub until free chlorine has dropped back to your normal level.
6. Rebalance the water and refit the mineral cartridge.

While the pipe cleaner circulates, expect foam and flecks of debris to appear on the surface. This is normal and shows biofilm is loosening from the pipes. Skim the debris off as it appears.

For the filter clean, use a dedicated filter cleaner spray and then rinse the filter thoroughly. The debris loosened from the pipes collects in the filter, and a dirty filter will release it straight back into the water.

Full drain and clean

To fully remove biofilm, most technicians recommend a pipe cleaner before each drain. If foam and cloudiness persist after a pipe cleaner and shock treatment, a full drain, clean and refill is the reliable fix.

1. Add a hot tub pipe and line cleaner to the water, following the dose on the label.
2. Run the jets for the time stated, usually with the cover off.
3. Switch off at the isolator and drain the tub completely.
4. Clean the shell and rinse or replace the filter.
5. Refill, then balance pH, alkalinity and calcium hardness.
6. Fit a new mineral cartridge and bring your sanitiser to its usual level.

After any biofilm treatment, replace the filter rather than just cleaning it if it is more than about 12 months old. An old, dirty filter can hold enough contamination to bring the problem straight back into clean water.

Keeping body oils down

Floating one or two clean tennis balls in the tub between soaks helps soak up body oils and lotions from the surface. This reduces the greasy film at the waterline and takes some of the load off your shock. Replace the balls when they look discoloured, and take them out before bathing.

Removing those oils matters for biofilm too. Body oils and lotions left on the water surface feed biofilm growth as well as using up your sanitiser. With a lower chlorine or bromine level in a mineral system, cutting that food supply between soaks is especially worthwhile.

Summary: what minerals really do

A hot tub mineral sanitiser is a useful helper, not a replacement. Silver holds back bacteria, copper holds back algae, and together they let you run a lower, steadier level of chlorine or bromine. You still need to test, balance and shock.

Choose a spa-rated cartridge sized for your tub, confirm it suits your sanitiser and warranty, and replace it on time. Keep pH between 7.2 and 7.6 to avoid copper stains. Done that way, minerals can make hot tub care a little calmer and the water more pleasant to sit in.

Frequently asked questions

Can I use a mineral cartridge with bromine?

Many mineral cartridges work with bromine as well as chlorine, but not all of them do. Some are designed only for chlorine. Check the packaging or the maker's instructions before you buy. If it is approved for bromine, run bromine at the level the cartridge maker gives, and test before each use, as levels can drop quickly after heavy bathing.

Can I add a mineral cartridge to water that already has chemicals in it?

Yes, as long as your current sanitiser is chlorine or bromine and the water is balanced. Test pH, alkalinity and sanitiser first and correct them, then fit the cartridge. You do not need to drain. If you currently use biguanide, you will need to drain, clean and switch sanitiser before fitting minerals, as the two are generally not compatible.

Is a mineral sanitiser the same as shock?

No. A mineral sanitiser releases silver or copper slowly over months to hold back bacteria and algae. Shock is a strong oxidiser, added weekly or after heavy use, that breaks down sweat, oils and other organic waste. Minerals cannot do that job, so you need both. Many mineral systems recommend a non-chlorine shock, but check your instructions.

Will minerals turn my hair green?

They can if the cartridge contains copper and the water has too much copper or a high pH. This mostly shows in blonde, grey or chemically treated hair. A silver-only cartridge greatly reduces the risk. Keeping pH between 7.2 and 7.6, rinsing your hair after bathing, and using only one mineral product also help prevent it.

Should I take the cartridge out when I shock or drain?

Most cartridges can stay in place during a normal weekly shock. Remove it before using a pipe and line cleaner or doing a drain, as strong cleaners can shorten its life. Many makers suggest fitting a new cartridge after each refill anyway. If your instructions say something different, follow them.

Do minerals mean I can test less often?

No, if anything you should test more carefully. Because you run a lower chlorine or bromine level with minerals, it can drop to zero faster after a busy session. Test sanitiser and pH before each use and at least three times a week. Water that looks clear is not proof that it is properly sanitised.