How do you measure the chlorine level? I bought some fake test strips on AliExpress, and they provided absolutely strange figures—like I was swimming in pure acid, but the water did not even smell. I decided to buy a chlorine test kit, and everything looked fine. I did everything right!

Are you sure that you have added the correct amount of chlorine to your pool? If you add chlorine dissolved in water, you need to check the concentration and adjust it to your pool's volume. Another reason, I think, could be the heat. You have an outdoor pool; maybe chlorine goes away with the water during the hot summer.

For me, it looks like you have a leakage in the plumbing. And you add fresh water, while water with chlorine goes into the ground? The leakage is not so big that you do not notice the increased water consumption. In this case, somewhere near your pool should be dead grass or other signs of chemical interference.

Your water is unbalanced. You need to check all water parameters, especially CYA. Your chlorine reacts with other chemicals in your pool. Test your overall pH; it should not be too high or too low. If you use unstabilized chlorine, it breaks down very quickly in sunlight. Change the chlorine type you add.

Chlorine, like many other sanitizers, keeps the water safe for swimming without germs. The normal chlorine level should be around three ppm, and you need to add the fresh chlorine only once or twice per week. There are several more reasons (in addition to what our readers already provided) why your chlorine level can drop: many organic and inorganic contaminants like leaves, branches, soaps, and shampoos in your water (please clean the pool); heavy rains (just add more chlorine after rains); and too much sunlight. You can perform a special test for unseen organics in your pool at night – the FAS/DPD test (is the must for combined chlorine levels below 0.5 ppm). After the test has shown that you have organics, you need to clean the pool, scrub the pool walls and floor and shock the pool.