Hello! I ama fairly new pool owner (July 2012) and have a pool waiter problem that no one so far has a solution to!
Stumbled across this website while researching pool chemistry. Hoping you can help!
POOL STATS: (I tried to edit my profile but said I didn't have permission until I posted?)
15,000 SF rectangular, in-ground w/ vinyl liner
Pentair ¾ HP self-priming pump
Pentair Clean & Clear Plus Cartridge Filter. Model# CCP240.
Nature2 Mineral Sanitizer: Professional G Vessel
In-line chlorinator
Heater (gas)
Automatic Pool Cover

I keep the pool warm (80-86 this season).
I close the pool (automatic pool over) nightly, open again by 9-10am.

Chlorine Supplies: 3” tablets (GLB or Doheny’s brand):
Trichloro-s-Triazinetrione: 99%, Inert Ingredients: 1%, Available Chlorine: 90%
Superchlorinator (to shock the pool): GLB Super Charge: 68% Calcium Hypochlorite

My problem began about 4 weeks ago. I noticed that Chlorine levels (FC and TC) were steadily dropping- from 1 to .5 to zero. I normally keep FC level at 1-2 was told I didn't need to go higher since I nada mineral sanitzer and CL demand would be low.
WHen TC and FC both reached zero for several days I texted my pool guy (pool installer) and he recommend getting water tested by pool store (The Great Escape) and shocking the pool with 2 lbs shock powder.

Water testing yielded strange results: FC=0. TC=0 pH=6.5 TA=24 CYA=115, hardness =199 no metals present, HIGH phosphates present )the exact number was not given in the report).
My test rests showed FC=0 TC=.5 pH=7.2 TA=110 (I didn't have tests for other criteria)

They recommended: shock w/ 2 lbs "Natures' Way Super shock, add 22 lbs alkalinity plus, add 1 1/2 lbs pH up, add 1.5 Liters commercial strength Phos free.

I started with shock: last Friday I shocked my pool with 2 lbs Lithium based shock (they said it was the most popular type and wouldn't stain my vinyl liner. Zero chlorine reading the next morning.
Saturday evening I tried chlorine based "super shock". Zero Chlorine reading the next morning.
I didn't believe their pH and TA reading were correct so I added only phos-free on Sunday. Within an hour the filter pressure soared from 15 to 28 and water circulation slowed dramatically, and the water clouded. The next morning I barely had enough suction to vacuum up the white residue on the bottom of the pool. The pool water was once again crystal clear.

I remember my pool guy telling me that sometimes fertilizer could mess up pool chemistry and incidentally I had my lawn treated at the same time my FC levels started dropping. He suggested the pool might be over chlorinated and bleaching the free chlorine results and suggested I shock the pool again, test again for phosphates. He's not sure why shocking the pool is not raising my FC level. ARGH!!!

It's now Wednesday evening and even with reduced water flow, gunked up filters and zero FC in the water for several weeks now, the pool looks great.

Well, I am still waiting for my new filter cartridges to come in- i want my pool guy to install them only so that I can learn how to do them myself next time. (I'll admit the warning in the filter manual intimidates me…"These instructions MUST BE FOLLOWED EXACTLY to prevent the lid from blowing off during system restart or later operations"…)
Here's the latest water testing results that I took today:
FC=0
TC=0
TA=130
Hardness=750
pH=7.2
phosphates=500-1000 (did this test 3 times- dark blue but difficult to discern which color exactly)
CYA=way over 100 (the test vial (Taylor) only went up to 100, but the cloudy liquid measured 1 1/2 cm tall in the vial if that means anything.

Hopefully I'll have new cartridges tomorrow, BUt after reading about the CYA and Chlorine connection I think I understand that the amount of shock rehired to unlock the combined chlorine level needs to be much much much higher that the 2 1/2 + 2 lbs of shock that I added last week. SO why hasn't any pool guys or pool store guys mentioned this fact to me????
The kids are so bummed - they see a beautiful clear pool and don't understand how it can be so d dirty they can't swim in it….HELP!