You’ve lifted the cover and the water looks… off. Not green exactly, but not the sparkling clear you remember from last weekend. There’s a faint smell that wasn’t there before. Your test strips are showing combined chlorine through the roof. What you’re looking at is a hot tub that needs shocking — and putting it off only makes things worse.
In This Article
- What Shocking Actually Does
- When to Shock Your Hot Tub
- Chlorine Shock vs Non-Chlorine Shock
- What You’ll Need
- Step-by-Step: How to Shock a Hot Tub
- How Long to Wait Before Using the Tub
- Shocking a Bromine Hot Tub
- How Often Should You Shock
- Common Shocking Mistakes
- Troubleshooting After Shocking
- Safety Precautions
- Frequently Asked Questions
What Shocking Actually Does
Shocking is the process of adding a large dose of oxidiser to your hot tub water. The purpose isn’t to “clean” the water in the scrubbing sense — it’s to break down organic contaminants that regular sanitiser can’t handle.
Breaking Down Chloramines
When chlorine or bromine does its job — killing bacteria, breaking down sweat, oils, and dead skin cells — it creates waste products called chloramines (or bromamines). These are the compounds that give hot tubs that harsh chemical smell. Contrary to what most people think, a strong chlorine smell doesn’t mean there’s too much chlorine. It means there’s too much used-up chlorine that needs clearing out.
Shocking oxidises those chloramines and converts them back into free chlorine that can actually sanitise. Think of it as resetting your sanitiser.
Killing Resistant Organisms
Some bacteria and biofilm are tough enough to survive normal sanitiser levels. The HSE recommends regular shock dosing as part of legionella prevention — a real concern in warm water systems like hot tubs. A proper shock raises the oxidiser level high enough to kill organisms that shrug off standard doses.
Clearing Organic Build-Up
Body oils, lotions, deodorant residue, and even dead skin cells accumulate faster than you’d expect in 1,000-1,500 litres of warm water. Four people using a hot tub for 30 minutes can introduce a surprising amount of organic matter. Shocking breaks this down before it turns into a breeding ground.
When to Shock Your Hot Tub
Regular Schedule
Once a week is the baseline for a hot tub that gets used 2-3 times per week. If you’re using it daily, twice a week is sensible.
After Heavy Use
Had a party? Four or five people in the tub for an extended session? Shock immediately afterwards. The contaminant load from multiple bathers overwhelms normal sanitiser levels.
After Refilling
Always shock fresh water after a drain and refill. Even mains water contains trace organic compounds, and the plumbing retains biofilm that needs blitzing.
When the Water Smells or Looks Off
Cloudy water, foam that won’t dissipate, or that characteristic “chlorine pool” smell are all signs that chloramines have built up. Don’t add more regular sanitiser — shock it. If you’re dealing with persistent cloudiness, our cloudy water troubleshooting guide covers the full range of causes.
After Extended Non-Use
Going on holiday for two weeks? Shock before you leave and shock again when you return. Stagnant warm water is a bacterial paradise.
Chlorine Shock vs Non-Chlorine Shock
This is the most common point of confusion for hot tub owners, and it matters because they do different things.
Chlorine Shock (Dichlor / Sodium Dichlor)
- What it is: A high dose of granulated chlorine (usually sodium dichloroisocyanurate)
- What it does: Sanitises AND oxidises — kills bacteria, viruses, and algae while also breaking down chloramines
- When to use: After heavy use, when water is visibly dirty or smelly, after a refill, or anytime you suspect bacterial contamination
- Downside: Adds to your cyanuric acid (CYA/stabiliser) levels over time. CYA above 50ppm locks up chlorine and makes it less effective — at that point, a water change is needed
- Wait time: 12-24 hours before bathing (until free chlorine drops below 5ppm)
Non-Chlorine Shock (MPS / Potassium Monopersulphate)
- What it is: An oxidiser that doesn’t contain chlorine
- What it does: Oxidises organic waste and chloramines but does NOT kill bacteria or viruses
- When to use: Weekly maintenance shocking when the water is already clean and well-sanitised. It clears the gunk without adding more chlorine
- Advantage: Doesn’t affect CYA levels, and you can usually use the tub 15-20 minutes afterwards
- Downside: Won’t fix a contamination problem on its own — if the water is green or smelly, you need chlorine shock
Which Should You Use?
Both. Use non-chlorine shock for routine weekly maintenance — it clears organics without pushing your CYA up. Use chlorine shock when you need actual sanitisation power: after heavy use, after a refill, or anytime something’s gone wrong.
What You’ll Need
- Shock treatment — either chlorine granules (dichlor) or non-chlorine shock (MPS). About £8-15 for a 1kg tub from Amazon UK, Hot Tub Suppliers, or your local pool shop
- Test strips or a liquid test kit — to check pH, free chlorine, and total alkalinity before and after. Strips are about £8-12 for 50 at Amazon UK
- Protective gloves — chemical-resistant gloves, not rubber washing-up gloves
- Safety goggles — any splash-proof eye protection
- A measuring cup or scoop — most tubs include one, but a dedicated kitchen measuring cup works
- Your hot tub manual — for the recommended dosage per 1,000 litres (varies by brand)
Step-by-Step: How to Shock a Hot Tub
- Test the water with strips or a liquid kit — note your pH, free chlorine, total chlorine, and alkalinity
- Adjust pH to between 7.2 and 7.6 if it’s outside that range — shocking at high pH reduces effectiveness
- Remove the hot tub cover completely — gases need to escape during shocking
- Turn on the jets and blowers to maximum — you want full circulation so the shock distributes evenly
- Measure the correct dose of shock treatment according to your product’s instructions (typically 15-20g of dichlor per 1,000 litres, or as per the label)
- Sprinkle the granules directly onto the water surface while the jets are running — don’t dump it all in one spot
- Leave the jets running for at least 20-30 minutes to distribute the treatment thoroughly
- Leave the cover off for at least 30 minutes after shutting off the jets — the off-gassing chloramines need to escape
- Wait the appropriate time before bathing — 15-20 minutes for non-chlorine shock, or until free chlorine drops below 5ppm for chlorine shock (usually 12-24 hours)
- Re-test the water before getting in to confirm safe sanitiser levels
Getting the Dosage Right
More isn’t better. Overdosing chlorine shock doesn’t speed things up — it just means waiting longer for levels to drop. Follow the product instructions and your tub manual. A typical 1,000-litre hot tub needs about 15-20g of dichlor for a regular shock, or up to 40g for a heavy shock after contamination.
Why pH Matters
Chlorine works best between pH 7.2 and 7.6. At pH 8.0, chlorine is less than half as effective. Shocking at high pH is wasting product and your time. Always test and adjust pH first.
How Long to Wait Before Using the Tub
After Non-Chlorine Shock
15-20 minutes with the cover off. MPS doesn’t contain sanitiser, so once it’s dispersed and the water has circulated, you’re safe. Test to confirm the oxidiser has dissipated if your kit measures MPS levels.
After Chlorine Shock
This depends on how much you added and your tub’s volume. The safe threshold is free chlorine below 5ppm — most health guidelines use this figure. After a standard shock dose, that’s typically 12-24 hours. After a heavy shock (double dose for contamination), it can take 24-48 hours.
Testing Before Entry
Don’t guess. Dip a test strip before getting in. If free chlorine reads above 5ppm, wait. If it’s between 3-5ppm, you’re fine. Below 3ppm means the shock has done its job and dissipated.
Shocking a Bromine Hot Tub
If you use bromine instead of chlorine as your primary sanitiser, the process is slightly different.
Can You Use Chlorine Shock in a Bromine Tub?
Yes — and it’s actually recommended. Chlorine shock (dichlor) activates spent bromine in the water, converting bromide ions back into active bromine sanitiser. This is called reactivation, and it’s why many bromine hot tub owners use dichlor shock rather than MPS.
Non-Chlorine Shock With Bromine
MPS also reactivates bromine, so both types work with bromine systems. The choice comes down to whether you need sanitising power (chlorine shock) or just oxidation (MPS).
Bromine Shock Frequency
Same as chlorine: weekly for regular use, plus after heavy bather loads. Bromine produces bromamines instead of chloramines — they’re less smelly but still need clearing out.
How Often Should You Shock
Weekly Maintenance
- Light use (1-2 people, 2-3 times per week): Once per week with non-chlorine shock
- Moderate use (3-4 people, 3-4 times per week): Once per week with non-chlorine shock, plus a chlorine shock every 2 weeks
- Heavy use (daily, multiple bathers): Twice per week — alternate between non-chlorine and chlorine shock
Seasonal Adjustments
In summer, bacteria grow faster and bather loads tend to increase. Shock more frequently — every 4-5 days instead of weekly. In winter, if the tub sits unused for stretches, shock before and after each period of non-use.
After Specific Events
- Party with 6+ bathers: Chlorine shock immediately after
- Children in the tub: Young children introduce more organic matter — shock after extended children’s sessions
- After illness: If anyone who’s been ill used the tub, chlorine shock to be safe
- Water change: Always shock fresh fill water
Common Shocking Mistakes
- Not removing the cover — the gases produced during shocking need to escape. Leaving the cover on traps them, which damages the cover underside and reduces the shock’s effectiveness
- Shocking at high pH — chlorine is less than half as effective above pH 7.8. Test and adjust pH first, then shock
- Adding shock through the skimmer — concentrated chemicals can damage the pump, heater, and plumbing. Always add directly to the water surface
- Using the tub too soon — “I only added a bit” isn’t a reliable safety measure. Test the water. Below 5ppm free chlorine, or you wait
- Mixing different chemicals — never mix chlorine and bromine products in a container or add them to the water at the same time. Chemical reactions between different products can produce toxic gases
- Not running the jets — shock needs circulation to distribute evenly. Dead spots in the water won’t get treated if the jets are off
- Storing chemicals improperly — keep shock products in a cool, dry place away from direct sunlight. Never store chlorine and bromine products next to each other
If you’re new to hot tub chemistry in general, our beginner’s guide to hot tub chemicals covers the foundations before you get into shock treatments.
Troubleshooting After Shocking
Water Still Cloudy After Shocking
If the water doesn’t clear within 24 hours of a chlorine shock, the problem might not be organic contamination. Check:
- Filter condition — a clogged filter can’t clear the particles a shock releases. Clean or replace it
- Total alkalinity — if it’s above 150ppm, the water struggles to clear. Adjust with pH decreaser
- Calcium hardness — above 400ppm causes cloudiness. This needs a partial water change
Chlorine Level Won’t Drop
If free chlorine is still above 10ppm after 48 hours, you’ve likely overdosed. Options:
- Wait it out — leave the cover off and run the jets. UV from sunlight breaks down chlorine
- Use a chlorine neutraliser — sodium thiosulphate (available from pool shops) drops chlorine levels quickly
- Partial water change — drain 25% and refill with fresh water
Foam After Shocking
Foam usually means the shock released a lot of built-up organic matter (body oils, lotions). Skim the foam off, run the jets for 30 minutes, then check again. If it persists, you might need a dedicated anti-foam product or a full water change.
Strong Chemical Smell Persists
If the chloramine smell doesn’t clear after 24 hours, the shock dose wasn’t strong enough. Run a second chlorine shock at the standard dose and leave the cover off for at least an hour.
Safety Precautions
Chemical Handling
- Always wear gloves and eye protection when handling shock products
- Never mix chemicals — add one product at a time and allow 15 minutes between different treatments
- Add chemicals to water, never water to chemicals — this applies when pre-dissolving in a bucket too
- Wash hands thoroughly after handling any hot tub chemicals
- Keep chemicals away from children and pets
Storage
- Store in a cool, dry, ventilated area — not in the hot tub cabinet where heat accelerates degradation
- Keep containers sealed — moisture causes granules to clump and lose potency
- Never return unused chemical to the original container — contamination can cause reactions
Emergency
If you accidentally get shock treatment in your eyes, flush with clean water for 15 minutes and seek medical advice. If ingested, call 999 or Poison Control immediately. Keep the product container for reference.
For a broader overview of every chemical you need, our hot tub chemicals glossary explains all the key terms.
Frequently Asked Questions
Can I shock my hot tub with bleach? No. Household bleach (sodium hypochlorite) has additives and an inconsistent concentration that makes it unsuitable for hot tubs. Use a product specifically designed for hot tubs or pools — sodium dichlor for chlorine shock or potassium monopersulphate for non-chlorine shock. The cost difference is minimal and the safety difference is significant.
Can I shock my hot tub and use it the same day? With non-chlorine shock (MPS), yes — typically within 15-20 minutes. With chlorine shock, you need to wait until free chlorine drops below 5ppm, which usually takes 12-24 hours. Always test before getting in.
How much shock do I need for a 1,500-litre hot tub? For dichlor (chlorine shock), about 22-30g for a routine weekly shock — roughly 1.5 tablespoons. For a heavy shock after contamination, double that. For non-chlorine shock (MPS), follow the product label but typically 30-40g per 1,500 litres. Always check your specific product’s dosing instructions.
Does shocking a hot tub raise the pH? Non-chlorine shock (MPS) is acidic and tends to lower pH slightly. Chlorine shock (dichlor) is roughly pH-neutral but can shift readings temporarily. Always test pH after shocking and adjust if needed before using the tub.
Should I shock before or after adding fresh chlorine? Shock first, then test the next day and add regular sanitiser to maintain your target level. Shocking resets the water chemistry — adding chlorine first and then shocking wastes the chlorine you just added.