Hot Tub Water Treatment Guide — How to Keep Your Water Clean

Water treatment is one of the most misunderstood topics in the hot tub world. We regularly hear from customers who think a filter alone is enough, who confuse modern water treatment with old-fashioned chlorine, or who simply don’t know where to start. That’s why we decided to write this guide — to clear up the confusion once and for all.

The good news: keeping your hot tub or ice bath water clean is simpler than most people think. You don’t need a chemistry degree or a cabinet full of products. You need a filter, the right chemical treatment, and a five-minute routine.

The Simplest Hot Tub Water Care Routine

If your hot tub uses continuous filtration, the simplest approach is to replace the water approximately once a month and follow the dosing schedule below. With continuous filtration, showering before use and correct dosing, this keeps water care straightforward for most private users.

You can keep the same water for longer than one month if it remains clean and well maintained. In that case, we recommend monitoring pH and disinfectant concentration with appropriate test strips or a photometer and adjusting the dosing accordingly.

Heavy use, many bathers, or cloudy, foamy or unusual-smelling water may require earlier water replacement regardless of how long the water has been in the tub.

The core routine:

1. Shower before use.
2. Run filtration regularly / continuously according to the filtration system.
3. Follow the TNS dosing schedule.
4. Check pH regularly.
5. Replace the water approximately once per month.

Filtration and Chemical Treatment — The Two Pillars of Clean Water

Hot tub water treatment consists of two things. Both are essential. One does not work without the other.

1. Physical filtration — removes what you can see

A filter catches dust, skin particles, body hair, insects, pollen, and any other solid debris that ends up in the water. Without a filter, your water will turn cloudy and dirty regardless of what chemicals you add.

But a filter alone is not enough. A filter does not kill bacteria. It does not prevent algae. It does not stop biofilm from growing inside pipes and hoses. For that, you need the second pillar.

2. Chemical treatment — controls what you can’t see

Bacteria, algae, biofilm, yeast, and fungi are invisible to the naked eye but thrive in warm water (33–40 °C). Without chemical treatment, they will take over your hot tub within days — even if your filter is running perfectly.

Chemical treatment keeps these microorganisms under control so your water stays hygienic, clear, and safe to bathe in.

A simple habit that makes a big difference: shower before you get in

A quick shower before entering removes a significant amount of sweat, body oils, cosmetics, dead skin and other contaminants that would otherwise enter the water. This reduces the load on the disinfectant and helps keep the water clear for longer.

The result: cleaner water, fewer chemicals needed and a longer interval between water changes. A quick shower before every soak is the single easiest thing you can do to keep your water in good condition.

Don’t forget your feet. If you walk across the lawn or garden to reach your hot tub, your feet pick up grass, dirt, and soil — all of which ends up in the water. Use a foot bath next to the tub to rinse your feet before stepping in, or wear flip-flops on the way over.

Hot Tub Chemicals: Why We Don’t Use Chlorine

In the past, chlorine was the standard for water treatment. Traditional chlorine-based treatment is not recommended or permitted in TNS hot tubs because prolonged exposure can be too aggressive for some of the materials, metal components, fittings and seals used in our systems.

Today, water treatment is done with modern compounds that are gentler and well suited to hot tub use:

• Active oxygen — skin-friendly, practically odourless
• Chlorine dioxide — not to be confused with chlorine! Despite the similar name, chlorine dioxide is a completely different compound. It is gentle on skin, inhibits biofilm, and breaks down relatively quickly after treatment.
• Bromine — a powerful and very robust disinfectant that performs particularly well in warm spa water, effective against bacteria, viruses, and fungi

When used correctly and according to the recommended concentration, all three are well suited to hot-tub water treatment and generally have much less noticeable odour than traditional chlorine treatment. This guide explains how to use each one.

Hot Tubs and Ice Baths Need Different Care

Hot water (33–40 °C) is a breeding ground for bacteria, biofilm, yeast, and fungi. At these temperatures, microorganisms multiply quickly, which is why hot tubs require both filtration and chemical treatment at all times.

Cold water (around 4 °C) significantly slows the growth of many microorganisms, which means an ice bath generally requires much less chemical treatment than a hot tub. Algae is often the most visible problem, particularly when the bath is exposed to sunlight, but regular cleaning, filtration and water replacement are still important.

Hot Tub Water Treatment

Active Oxygen, Chlorine Dioxide or Bromine — Which One to Choose

There are three practical disinfectant options for home hot tubs. Pick one. Do not mix them.

Option 1: Active oxygen (MPS / KHSO₅ tablets) — our recommendation
The gentlest option on skin and practically odour-free. Ideal for people with sensitive skin or eczema. Active oxygen requires a well-balanced water chemistry — it works best when pH is kept around 7.0 and paired with a small weekly dose of algaecide. Active oxygen tablets are generally easy to store, but they should still be kept according to the storage instructions on the packaging.

→ Buy active oxygen tablets

Option 2: Chlorine dioxide tablets
A strong all-round disinfectant that also inhibits biofilm — the slimy buildup that can form inside pipes and hoses. Chlorine dioxide works across a wider pH range (6.5–7.5) than most alternatives and breaks down relatively quickly after treatment without behaving like traditional chlorine in the water. One tablet per week is enough. Important: chlorine dioxide tablets come in foil blister packs (similar to medicine tablets) and must be stored in a dry place — they are sensitive to moisture and will be ruined if left outside or in damp conditions.

→ Buy chlorine dioxide tablets

Option 3: Bromine (BCDMH tablets)
Bromine is a powerful and very robust disinfectant that performs particularly well in warm spa water. It is effective against bacteria, viruses and fungi. Bromine is forgiving and works well even if water balance isn’t perfect. Slightly more aggressive on skin than the other two options, although many users notice little difference.

Interested in bromine treatment? Leave your contact details and we’ll let you know as soon as it’s available.

Which one should you choose?

	Active oxygen	Chlorine dioxide	Bromine
Skin sensitivity	Very gentle	Gentle	Slightly more aggressive
Smell	Almost none	Minimal	Minimal
pH sensitivity	Needs precise pH (~7.0)	Wide range (6.5–7.5)	Forgiving (7.0–7.5)
Biofilm performance	Basic	Excellent	Good
Storage	Easy — store according to label	Must stay dry (foil pack)	Easy — store according to label

Our recommendation: always start with active oxygen. It’s gentle, effective, and works for the vast majority of hot tub owners — whether you use the tub at home or run a rental property. If active oxygen alone isn’t keeping your water clean (for example, the water still goes cloudy even when dosing two tablets per week), then it makes sense to switch to chlorine dioxide or bromine for stronger protection. But start with active oxygen first — most people never need anything else.

Dosing — Active Oxygen (MPS 20 g Tablets)

Add one tablet per dose. You can drop active oxygen tablets directly into the tub — they will not damage the surface. For best results, also add approximately 20 mL of liquid algaecide per week as a backup against algae.

How often to add one tablet (20 g):

Tub volume	At 25–26 °C	At 33 °C	At 39–40 °C
~600 L	Every 6 days	Every 5 days	Every 4–5 days
~1,350 L (e.g. Nordic Six)	Every 4–5 days	Every 3 days	Every 3–4 days
~1,650 L	Every 4 days	Every 3 days	Every 2 days

For the Nordic Six (~1,350 L at ~39–40 °C): 2 tablets per week — approximately one tablet every 3–4 days.

These figures are practical starting doses rather than exact laboratory requirements. Because whole 20 g tablets are used, dosing intervals are intentionally rounded. Actual demand depends on water temperature, number of bathers, frequency of use and how much contamination enters the water.

A practical routine: Add one tablet before you use the hot tub (e.g. Friday evening), and another one three to four days later (Monday or Tuesday) to keep the water clean until your next session. Adjust based on your tub size and temperature — smaller or cooler tubs need less frequent dosing.

After a water change: Simply start the normal dosing routine, unless the specific product label states otherwise.

→ Buy active oxygen tablets (20 pcs)

Dosing — TNS Chlorine Dioxide 20 g Tablets

Add one tablet per week. That’s it — chlorine dioxide is the simplest dosing routine of the three options. The dosing below applies specifically to the TNS chlorine dioxide 20 g tablet product.

For the Nordic Six (~1,350 L at ~39 °C): 1 x 20 g TNS chlorine dioxide tablet per week.

How to add the tablet: The safest method is to dissolve the tablet first in a small container of water (e.g. a foot bath or bucket), then pour the solution into the tub. Chlorine dioxide is a powerful cleaner, and if a tablet sits directly on the tub surface while dissolving, it may clean that spot so thoroughly that it leaves a visible mark — a lighter patch where the tablet sat. This is not damage, just a difference in cleanliness, but it can be noticeable on coloured surfaces and is not covered by warranty. If you prefer convenience, you can drop the tablet directly into the water, but be aware of this possibility.

After a water change: Add one tablet immediately after filling to begin disinfection before the water heats up.

→ Buy chlorine dioxide tablets (12 pcs — enough for ~12 weeks)

Dosing — Bromine (BCDMH 20 g Tablets)

How to add the tablet: We recommend dissolving the tablet first in a small container of water (e.g. a foot bath or bucket), then pouring the solution into the tub. We have not yet tested the effect of bromine tablets dissolving directly on tub surfaces, so dissolving separately is the safer approach.

How often to add one tablet (20 g):

Tub volume	At 25–26 °C	At 33 °C	At 39–40 °C
~600 L	Every 8+ days	Every 5 days	Every 3–4 days
~1,350 L (e.g. Nordic Six)	Every 4–5 days	Every 3 days	Every 2–3 days
~1,650 L	Every 3–4 days	Every 2–3 days	Every 2 days

For the Nordic Six (~1,350 L at ~39–40 °C): approximately 1 x 20 g bromine tablet every 2–3 days as a practical starting point.

Target private-spa bromine concentration: approximately 3–6 mg/L.

The table provides a practical starting schedule. Bromine is best managed by checking the bromine concentration in the water and adjusting the dosing interval accordingly.

After filling with fresh water: Two tablets may be used as the initial dose to establish the bromine level more quickly. During normal maintenance, add only one tablet at a time.

Important: Add one tablet at a time rather than several tablets at once. If you need more disinfection, dose more frequently — not more at once.

Temperature matters: Bromine degrades almost twice as fast at 40 °C as it does at 26 °C. If you keep your hot tub at a lower standby temperature and heat up before use, your tablets will last significantly longer.

Interested in bromine treatment? Leave your contact details and we’ll let you know as soon as it’s available.

Dosing tables are practical starting recommendations for private use. Actual chemical demand varies with water temperature, bather load, bathing frequency, filtration, water quality and contamination entering the tub. Always follow the instructions supplied with the specific treatment product.

How to Balance Hot Tub pH

Most water problems come down to pH. Here’s how to stay on top of it:

1. Test pH at least once a week — more often with heavy use
2. Use pH+ (sodium carbonate) to raise pH if it’s too low
3. Use pH− (sodium bisulphate) to lower pH if it’s too high
4. Adjust in small amounts. Wait an hour. Retest.

pH chemicals are cheap — a 1.5 kg container of pH− costs a few euros and lasts for months. It’s always cheaper and easier to adjust pH than to change the water.

Why pH matters so much: Maintaining the correct pH is important for bather comfort, equipment protection and effective water treatment. Different disinfectants tolerate pH changes differently, but keeping the water around neutral is a good general target. Aim for pH 7.0–7.5 unless the specific product recommends otherwise. For active oxygen, staying close to pH 7.0 gives the best results.

How to Test Hot Tub Water

For the simplest private-use routine, regular pH testing together with the dosing schedule and approximately monthly water replacement is usually sufficient. If you keep the same water for longer, use the tub heavily, or use bromine, measuring the disinfectant concentration provides much better control.

The easiest way to check pH is with pH test strips — dip the strip, wait a few seconds, and compare the colour. They’re available at most pool supply stores and online. Disinfectant-specific test strips are also available for active oxygen, chlorine dioxide and bromine.

How Often to Change Hot Tub Water

Private home use with continuous / regular filtration: Approximately once per month is the simplest routine. This keeps water management straightforward and reduces the need for frequent testing.

Want to keep the same water substantially longer? That is absolutely possible if the water remains clean and well maintained. In that case, monitor pH and disinfectant concentration more actively and observe water clarity, odour and foam.

Heavy use, many bathers, or water that looks or smells off may require earlier replacement regardless.

Important: The approximately one-month recommendation assumes that the hot tub has a filtration system running continuously or according to the manufacturer’s recommended daily schedule. Without regular filtration, water should be replaced significantly more frequently.

Rental / accommodation: Weekly water changes are standard. With proper disinfection between guests, you can maintain quality through the week, but don’t stretch it further. If the water doesn’t look right before a guest arrives, change it — no chemical will fix it fast enough.

Hot Tub Temperature and Chemical Use

A detail worth knowing: all disinfectants degrade faster in hotter water. If you keep your hot tub at a lower standby temperature (e.g. 26 °C and heat up before use), your chemicals will last significantly longer and you’ll dose less frequently.

This is particularly relevant if you’re using a heat pump — maintaining a lower base temperature and heating up on demand is both energy-efficient and easier on your water chemistry.

Hot Tub Filter Maintenance

If your hot tub has an external filter system, run it continuously or according to the filtration system manufacturer’s recommended daily schedule. The filter handles particles; the disinfectant handles microorganisms. They work as a team — one without the other is not enough.

These water-change intervals assume that the hot tub has a filtration system running continuously or according to the manufacturer’s recommended schedule. Without regular filtration, water should be replaced significantly more frequently.

FilterMax maintenance: The FilterMax uses cotton filter balls instead of traditional cartridges. Wash the filter balls with every water change — at minimum once a month. The easiest way is to put them in a washing machine without detergent. If you do use detergent, use only a tiny amount — regular use of washing powder or liquid will damage the filter balls over time. This keeps the flow rate high and the filtration effective.

Don’t have a filter yet? If your hot tub doesn’t have built-in filtration, we have a patented solution for adding one. Our stainless steel hose fitting allows you to connect a FilterMax to virtually any existing hot tub or ice bath.

→ FilterMax — external filter for hot tubs and ice baths
→ Stainless steel hose fitting — add filtration to any tub

Important — wood-fired hot tubs: Most external filtration systems (especially their internal circulation pumps) are not designed to handle water temperatures above 39 °C. If you use filtration with a wood-fired hot tub, always shut off the filtration system before heating. Running the pump in water that is being heated above 39 °C can burn out the motor. Once you’re done bathing and the lid is back on, you can safely resume filtration as the water cools down.

Cleaning the bottom and seating area

Even with a filter running, debris settles on the bottom of the tub and in the seating area — sand, small particles, and sediment that the filter can’t reach. A cordless spa vacuum cleaner is the easiest way to clean these areas without draining the water.

This is especially useful in summer when more outdoor debris ends up in the tub, and for Airbnb and rental operators — you can quickly vacuum the bottom and seating between guests instead of doing a full water change every time. It takes a few minutes and keeps the tub looking fresh without wasting water.

→ NetSpa Cleaner — cordless spa vacuum

Ice Bath Water Treatment — Cold Plunge Maintenance

Cold water (around 4 °C) significantly slows the growth of many microorganisms, which means an ice bath generally requires much less chemical treatment than a hot tub. Algae is often the most visible problem, particularly when the bath is exposed to sunlight, but regular cleaning, filtration and water replacement are still important.

Weekly Routine — Private Use

For a lightly used private ice bath kept around 4 °C, our simplest routine is filtration, regular water replacement and a small weekly algaecide dose.

1. Add approximately 20–40 mL of liquid algaecide per week, depending on the specific algaecide concentration and product instructions. The exact product label always takes precedence.
2. Keep the bath covered when not in use — this limits algae growth and keeps debris out.
3. Rinse off before stepping in. A quick shower removes body oils and skin cells that would otherwise foul the water.

Shared or heavily used ice baths require a more robust hygiene and disinfection routine.

Heavy or Shared Use (Gyms, Wellness Centres)

Shared and commercial ice baths require more frequent monitoring, cleaning and water replacement. The correct routine depends heavily on bather load and local hygiene requirements.

Bromine or another approved disinfectant system may be suitable for higher-load installations, but this needs to be assessed on a case-by-case basis. Do not apply the simple private-user routine to commercial environments.

Water Changes

• Private, light use: Every few weeks. Change when the water looks or smells off.
• Shared or heavy use: Weekly, or after each session if the setting demands it.

UV and Ozone Water Treatment — Do They Work?

Some hot tubs and ice baths come with UV lamps or ozonators. Both are technically valid treatment technologies, but they come with practical considerations that are worth understanding.

UV systems can work when correctly sized, correctly installed, and properly maintained. However, there are several things to keep in mind:

• UV lamp output gradually decreases with age, even if the lamp still appears to be on.
• The quartz sleeve or lamp surface can become covered with scale, biofilm, algae or deposits, which reduces the amount of UV light actually reaching the water.
• A normal homeowner cannot easily tell by looking at the system whether effective UV output is still being delivered.
• UV lamps and quartz sleeves must be inspected and maintained according to the manufacturer’s instructions. Lamp output decreases over time and deposits on the sleeve can significantly reduce treatment effectiveness.
• UV only treats water while it passes through the UV chamber — it does not leave a disinfectant residual throughout the entire tub, hoses and plumbing.
• UV therefore cannot react directly throughout the tub when contamination is introduced by bathers.
• UV adds additional hardware, cost and maintenance compared with a simple tablet treatment system.

Ozone systems can also be effective when correctly sized and installed, but ozone output varies significantly between systems. Ozone adds equipment and maintenance and should not automatically be treated as a full replacement for filtration, cleaning and proper water management.

Our position: For most private hot tubs and ice baths, we prefer a simple filtration + tablet-based water-treatment routine because it is easier to understand, easier to maintain and more predictable in everyday use. With tablets, the customer knows when treatment was added and, when necessary, can verify the disinfectant concentration using test strips or a photometer. UV and ozone are not inherently unreliable, but their actual treatment performance is harder for a homeowner to verify.

Common Problems and Quick Fixes

Cloudy water
Cloudy water can be caused by incorrect pH, insufficient filtration, excessive organic contamination, low disinfectant levels or water that has simply been in use too long. Suggested troubleshooting order:

1. Check pH.
2. Check / rinse the filter.
3. Check treatment level if possible.
4. Run filtration.
5. If the water does not recover, replace it.

Green tint or slimy walls
Green water or slippery / slimy surfaces usually indicate algae growth. Brush the surfaces, check filtration, use the appropriate algaecide according to its instructions, and replace the water if the problem is severe.

Persistent foam
Caused by cosmetics, soap, body oils or organic contamination. Rinse before bathing. Use a foam reducer if needed. If foam continues to return despite filtration and treatment, the water needs replacing.

Strong chemical smell (bromine users)
A strong chemical smell can indicate that the water chemistry is out of balance or that the disinfectant is dealing with a high organic load. Check pH, check bromine concentration, clean / rinse the filter, and replace or refresh the water if necessary.

Skin or eye irritation
Check both pH and disinfectant concentration. Irritation can be caused by incorrect pH, excessive treatment concentration, contaminants or water that needs replacing.

Disinfectant doesn’t seem to last
Higher temperature, heavy use, organic contamination and incorrect water balance can all increase chemical demand. Check the water, clean the filter and shorten the dosing interval if necessary.

Hot Tub Chemicals Checklist — What to Keep in Stock

For a home hot tub:

• Active oxygen tablets or TNS chlorine dioxide tablets or bromine tablets — pick one
• pH+ and pH− powders
• pH test strips
• Liquid algaecide (essential for ice baths, useful as a hot tub backup with active oxygen)

For customers who want to keep the water longer than one month:

• Disinfectant-specific test strips (for active oxygen, chlorine dioxide or bromine)

For a rental or accommodation setup, add:

• A digital photometer for accurate measurements (Safe Swim Meter — coming soon to our shop)
• Larger stock of tablets
• An extra set of filter balls — swap in a clean set while washing the dirty ones
• A spa vacuum for quick cleaning between guests
• A FilterMax external filter if your tub doesn’t have built-in filtration

What Not to Do

• Don’t combine or switch primary treatment systems unless the products are specifically intended to be used together. If changing from one disinfectant system to another, replacing the water first is the safest and simplest approach.
• Don’t drop multiple tablets at once. Add one tablet at a time. If you need more disinfection, dose more frequently — not more at once.
• Don’t skip pH testing. Disinfectant only works when pH is in range.
• Don’t discharge chemically treated water directly into ponds, streams or other natural bodies of water. Follow the treatment product instructions and local disposal requirements.
• Don’t ignore cloudy or off-smelling water. It’s telling you something — test, treat, or change.

Need Help?

If your water repeatedly goes cloudy or green, you notice unusual smells, or you’re switching between disinfectant types and want guidance — reach out. We deal with this every day and we’ll help you sort it out.

Contact TheNordicSauna →

Use biocides safely. Always read the label and product information before use.

This guide is based on practical operating data for private hot tubs and ice baths, developed in consultation with water treatment specialists. Commercial and public-use installations may require tighter monitoring and more frequent water changes than described here.