Refilling a Hot Tub: What to Add First, and in What Order
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Adding sanitiser before balancing alkalinity and pH can render up to half of that expensive chemical inactive within the first hour. The sequence—not the products—determines whether your fresh fill stays clear or turns into a cloudy, smelly problem.
This chemical inefficiency explains why following the correct commissioning order saves money and prevents the common frustration of water that won’t behave no matter how much you dose.
The Cost of Wrong Order: Why Sanitiser First Fails
Fresh tap water arrives with its own pH and alkalinity levels but zero sanitiser protection. Sanitiser—chlorine or bromine—works by reacting with contaminants, but its effectiveness plummets when water chemistry is unstable.
When pH sits outside the ideal 7.2–7.6 range, sanitiser molecules get consumed fighting chemical instability rather than killing bacteria. You’ll add more product, see temporary improvement, then watch cloudiness return as the foundation remains unbalanced.
Following “add sanitiser immediately” advice often leads to this cycle: initial clarity, then cloudiness within days, followed by repeated dosing that costs more than the initial fill. Getting the foundation right first lets your sanitiser work efficiently from the start.
The Commissioning Sequence: Six Steps That Work
Based on Lay-Z-Spa’s guidance for setting up water after a refill, this sequence builds stability systematically. The order matters more than rushing to sanitise.
Step 1: Fill and circulate thoroughly
Run the pump and filtration system for several hours before adding any chemicals. This removes trapped air, circulates water through pipework, and provides a stable testing baseline. Rushing this step means treating water that hasn’t settled.
Step 2: Test alkalinity and pH
Use test strips to check your tap water’s starting point. Focus on total alkalinity first—the often-skipped foundation—then pH. Clearwater or AquaChek strips work well, though colour matching requires good light. Test twice if results seem borderline.
Step 3: Balance total alkalinity to 80–120 ppm
As Lay-Z-Spa recommends, total alkalinity acts as a pH buffer. Without proper levels, pH adjustments won’t hold. If low, add a TA increaser like Clearwater TA Plus (paid link). Skipping this leads to frustrating pH drift that wastes chemicals.
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Step 4: Adjust pH to 7.2–7.6
With alkalinity stable, pH can be fine-tuned using pH Plus or Minus. Add small amounts, retest, then adjust further if needed. This incremental approach beats single large doses that often overshoot the target.
Step 5: Add sanitiser
Only now—with balanced water—add your chosen sanitiser. Clearwater Chlorine Granules (paid link) work quickly for fresh fills. For those preferring bromine, Safe Splash Bromine Tablets (paid link) offer gentler sanitation. Properly balanced water makes either choice more effective.
View Clearwater Chlorine Granules on Amazon → (paid link)
Step 6: Shock dose to complete the reset
Lay-Z-Spa’s chemical guide specifies shocking after every refill to address bacteria introduced during filling. Non-chlorine shock products like Aquasparkle Non-Chlorine Shock clear water without adding sanitiser load.
- Fill tub and circulate 4+ hours
- Test alkalinity and pH
- Balance alkalinity first
- Adjust pH to target range
- Add sanitiser
- Shock dose per product instructions
This sequence becomes your refill routine. Get the Free Hot Tub Water Checklist to keep beside your chemical shelf.
Sanitiser Choice: Chlorine vs Bromine for Fresh Starts
Your usage pattern determines which sanitiser fits best on a fresh fill.
Chlorine granules like Clearwater Chlorine Granules (paid link) act quickly, making them strong for initial sanitisation. They’re cost-effective but require more frequent testing and can cause skin irritation for some users.
Bromine tablets such as Safe Splash Bromine Tablets (paid link) work more gently, better suiting regular users with sensitivity concerns. Bromine maintains effectiveness at higher temperatures but typically costs slightly more.
Buy Safe Splash Bromine Tablets on Amazon → (paid link)
For one-off refills where speed matters and chlorine tolerance isn’t an issue, granules work well. For daily use with comfort priorities, bromine tablets make more sense. Don’t combine both—they don’t work together efficiently.
Essential Tools for Reliable Refills
Three items ensure your sequence works as intended:
Test strips provide the measurements needed for precise balancing. While colour matching can be tricky, testing twice confirms readings. Both Clearwater and AquaChek strips serve well.
pH adjusters require patience—add half the recommended dose, retest, then adjust further. This prevents overshooting your target range.
Filter consideration: A clean or new filter prevents cloudiness that chemicals can’t fix. If your current filter has seen heavy use, replacing it during refill avoids undermining your chemical efforts.
The Science Behind the Sequence
This order builds water stability logically. Total alkalinity buffers pH swings, creating a foundation for precise pH adjustment. Only then does sanitiser work at peak efficiency.
Adding sanitiser to unbalanced water is like painting a damp wall—you’ll use more product for poorer results. UK maker guides assume this sequence because their dosing charts depend on stable water chemistry.
Common Refill Questions
How long should the pump run initially?
Circulate water for several hours before adding chemicals. This ensures water stabilises and air pockets clear, providing reliable test results.
Can I add chemicals together?
No—simultaneous addition causes reactions that neutralise products. Follow the sequence with brief intervals between steps.
Is shocking necessary after every refill?
Yes, according to UK guidance. Shocking addresses bacteria introduced during filling and resets the water completely.
What if water stays cloudy after following all steps?
Check your filter first—a clogged filter is a frequent cause of persistent cloudiness. Replace if worn, then retest. If problems continue, a water change may be needed.
Chemical Order Summary
| Step | Action | Key Point |
|---|---|---|
| 1 | Fill and Circulate | Run pump 4+ hours before chemicals |
| 2 | Test Water | Check alkalinity then pH |
| 3 | Balance Alkalinity | Target 80-120 ppm first |
| 4 | Balance pH | Adjust to 7.2-7.6 range |
| 5 | Add Sanitiser | Only after pH stable |
| 6 | Shock Dose | Recommended post-refill |
| 7 | Verify Filter | Replace if clogged or old |
Mastering the Refill Routine
The refill sequence you’ve just learned applies each time you change water. Now you understand the order, our water-change frequency guide shows how often you’ll need to run through these steps—typically every 3-4 months depending on usage.
Keep this routine accessible—a phone photo or printed checklist works well. This process becomes automatic once you experience the difference between fighting cloudy water and enjoying clear results from day one.
Get the Free Hot Tub Water Checklist for a printable version of the commissioning sequence. For deeper guidance on specific products or readings, explore our related articles below.
Related Guides
This refill sequence is the one-time version of a process that repeats with every water change. For guidance on how often that cycle occurs, see How Often Should You Change the Water in an Inflatable Hot Tub? .
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Note: General guidance only, verify details with a qualified professional or official source.