23 August 2021
If shocking the pool will keep the chlorine level up for a couple of days, then it goes away, that tells me you have organics growing. Either it's an early algae bloom or other organics are in the water.
I would guess that the readings you provided above came from the pool store. If they are correct your CYA and Calcium Hardness levels may not be low. It depends on what type of pool you have (vinyl lined, gunnite/plaster or fiberglass) and how you chlorinate the pool, chlorine or a salt water generator.
Either way you need to raise the chlorine level to shock level and maintain that level continuously until the water holds chlorine. This is known as SLAMMING the pool.
The correct shock level for water with 35 ppm of CYA is 14 ppm of chlorine. You can do this with granular chlorine (calcium hypochlorite) or liquid chlorine. But not with tablets. Liquid chlorine is the easiest.
- If you are using a salt water generator to sanitize the water (and based on your post I don't think you are) you will need to turn it off.
Take the tablets out of the chlorinator or floater or wherever they are.
First: Raise the chlorine level to 14 ppm.
Next: Take frequent readings of the free chlorine level (not more frequently than every 2 hours or less frequently than twice a day, once in the morning around sunup and once in the evening around sundown)
After each reading, if the chlorine level has dropped below 14 ppm, add chlorine to raise it back to that point
When you get to the point where you aren't losing more than 1 ppm of chlorine overnight and the combined chlorine is .5 ppm or less and the water is clear, you can stop the slam and let the chlorine level drift back down to the 3 - 5 ppm range with 3 ppm being the absolute minimum. I keep my water in the 4-5 ppm range.
The pump should run 24/7 during the slam. The pool should be vacuumed and brushed daily. And the filter should be backwashed when the pressure tells you to.
If you use liquid chlorine to do this, slowly pour the amount needed to raise the chlorine level to 14 ppm in front of a return in the deep end of the pool...with the pump running.
If you use granular chlorine, dissolve the amount needed, about 4oz at a time in a 5 gallon bucket of water and slowly pour that into the pool in front of the return, again, with the pump running.
If you need to know how much to add, tell me what kind of chlorine you are using and how many gallons of water the pool holds and I will calculate it.
If you have any questions let us know.