Pool chlorine won't go up: what to do

Adding chlorine but the reading stays low? The most common causes (algae, sun, old product, expired test kit) and how to fix them.

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If you add chlorine and the next day the reading is still low, something is using up chlorine faster than you add it, or the reading is wrong. Check in this order:

1. Is the reading right?

• Expired reagents, or reagents stored in the heat, give wrong readings. Check the use-by date.
• Very high chlorine can bleach the reagent and look like zero. If you've just shocked the pool, wait a few hours and test again.

2. Is the product still strong?

• Liquid chlorine loses strength quickly, especially in the heat: after a few months it can hold far less chlorine than the label says.
• Granules that are open, damp or kept in the sun also weaken. Store them closed, somewhere cool.

3. Is the sun destroying the chlorine?

In an outdoor pool, the sun destroys most unstabilised chlorine within hours. Test the stabiliser (cyanuric acid): the ideal is 30 to 50 ppm. If it's close to zero, that's the problem.

4. Is something using up the chlorine?

Algae you can't see yet, leaves on the floor and a dirty filter use up chlorine non-stop. Try a simple test: measure chlorine in the evening, after sunset, and again early the next morning. If it dropped by more than 1 ppm overnight, something is still using it up.

1. Brush the walls and floor and vacuum the dirt.

2. Backwash the filter or clean the cartridge.

3. Shock the pool and keep the filter running.

4. Repeat the overnight test until chlorine stops dropping.

Don't keep raising the dose blindly day after day. Too much chlorine irritates skin and eyes, and with tablets or stabilised granules it builds up stabiliser in the water.