I am an experienced pool owner facing a problem that I have never before experienced: I have recurring algae which responds very well to shock - and then comes back a week later. The other symptom is the mystery of the disappearing chlorine, which I'm sure is related. I'll start from the beginning:
I have a 22K gal plaster IG pool about 15 years old in Tampa, Florida. I use a 100 Sq Ft PentAir Quad DE filter and Pentair VS Intelli-Flo pump which can push approximately 100gal per minute through the filter at low back pressure. The pump runs enough hours of the day to circulate the entire pool at least once per day. I use Tri-Chlor tabs for sanitizing and 73% Calcium Hypochlorite powder for shocking. I try to maintain levels as follows:
- pH normally settles at around 7.6 to 7.8. I apply small amounts of acid weekly to adjust pH closer to 7.4.
- TA should be in the 80-120ppm range and I consider 100ppm to be about perfect.
- CH should be around 220-240ppm
- CYA should be between 25-40ppm
As of a few minutes ago, pH=7.1, TA=60, CYA=75, CH=350 and CL is non-existent (no amount of DPD powder will make the test sample turn pink). I confirmed these tests at the local pool store and they had much the same results. Water is currently crystal clear, but there are signs of green algae visible in spots (might be dead and just not brushed loose yet). Water smells and looks great.
I understand that CH and CYA are both high and that I probably need to partially drain and refill the pool to correct both. The reason pH is low is due to the enormous amount of Chlorine I've used SLAMming the pool over the last few days. I have applied 9 pounds of 73% Cal Hypo shock powder since Sunday evening as follows: I thoroughly cleaned the filter (disassembled, scrubbed, fresh DE powder) on Sunday afternoon. I restarted the filter and applied three pounds Sunday evening around the pool perimeter and left filtration running. Twelve hours later (Monday morning), I applied another three three pounds and left the flitration running. Twelve hours later (on Monday evening) I added an additional three pounds of 73% Cal Hypo powder sprinkled around the perimeter while the pump was still running, waited about 30 minutes for distribution and then checked CL levels: 1.5ppm according to my test kit. By the next morning, CL was non-existent again. I thought my test kit might be bad and so took a sample to the pool store - they got the same results that I did and mentioned some BS about "Chlorine Lock". I should mention that Phosphates are well within range and TDS is a little high, but not out of range. it's been overcast, cloudy and rainy here for days, so sunlight/UV should have no more than a minor impact.
You read that last part right: three pounds of 73% Cal Hypo powder only raised CL to 1.5ppm after 30 minutes and was completely gone by the next morning. That's on top of the six pounds applied in the preceding 24 hours.
As mentioned above, I know that I need to at least partially drain and refill the pool to reduce CH and CYA levels. And the Cal Hypo that I am dumping in the pool doesn't appear to be accomplishing anything other than depleting my checking account and making the CH problem worse. So rather than continue on, I decided to develop a comprehensive plan and that's why I'm posting here. I appreciate your suggestions.
Should I continue adding mass quantities of chlorine in the hopes that I will eventually kill whatever invisible organics are consuming the chlorine? Or should I just bite the bullet, dump half the water, refill and balance and try again?