Quick answer
Test free chlorine, combined chlorine and CYA. Clean the filter and baskets, restore free chlorine to the range for your CYA, and run the pump long enough to distribute it. If CYA is very high, dilution may be the only reliable fix; check local discharge rules before draining.
What you're seeing
- Free chlorine reads zero or near zero soon after dosing.
- The pool is cloudy, slippery or starting to turn green.
- Chlorine tablets dissolve but the test stays low.
- You smell chlorine strongly even though the free reading is low.
What usually causes it
Cause 01
Organic demand is consuming it
Algae, leaves and biofilm consume chlorine faster than a maintenance dose can replace it. The pool needs a shock-level dose and cleaning, not another tablet.
Cause 02
CYA is too high
Stabilizer holds chlorine, but excessive CYA reduces its killing power. If CYA is very high, adding more chlorine may not solve the problem.
Cause 03
The feeder or salt cell is not delivering
A clogged feeder, depleted salt cell or failed injector can leave chlorine in the bucket instead of the pool.
Cause 04
Pump runtime is too short
Chlorine needs circulation to reach every corner. A pump that runs only a few hours a day leaves dead spots.
How to fix it, in order
Work through these steps once, with the pump running and the filter clean. Do not stack a second chemical dose before the first has circulated and you have retested.
- 01
Test FC, CC and CYA together
Free chlorine tells you what is available; combined chlorine tells you what is spent; CYA tells you how effective the free chlorine can be.
- 02
Clean the system before raising chlorine
Empty the baskets, brush the walls and clean the filter. Chlorine spent fighting debris will not show up as free chlorine.
- 03
Dose to the right target
Raise free chlorine to the maintenance or shock target for your CYA using a dose calculator. Add it with the pump running and retest after circulation.
- 04
Check the delivery equipment
Confirm the feeder is not clogged, the salt cell is clean and producing, and the pump run time is long enough for the pool volume.
- 05
Address high stabilizer honestly
If CYA is very high, partial dilution is the reliable fix. Follow GTA discharge rules and do not drain a vinyl-liner pool without professional advice.
What not to do
- Do not keep adding tablets without testing CYA; it can climb to a level where chlorine stops working.
- Do not shock and swim immediately; follow the label and wait until free chlorine is back in the safe range.
- Do not drain a vinyl pool to lower CYA without checking the risks and local rules.
Keeping it from coming back
- Test free chlorine two or three times a week in swim season.
- Keep CYA in the recommended range for your sanitizer and climate.
- Run the pump long enough to distribute chemicals and filter the volume.
- Shock after heavy use, storms or any period with the pump off.
When to stop and call a pro
Do not force it
Call a professional if chlorine will not register after a correct dose, CYA is very high, the salt system shows a fault, or the pool is already green. Repeated chlorine loss can also point to a hidden organic load or a failing component.
Compare local repair helpGeneral guidance for residential pools in Ontario. Chemical doses depend on pool volume and test results; follow the product label and keep gas, electrical and major shell work with licensed trades.