Unable to maintain FC

[elguapomp]

Ok everyone, I've been avoiding posting, but I'm out of options.

I have an inground pebble tec pool/spa that I believe is approximately 18k gallons. I typically dose chemicals based on 20k in case my math is off.

I drained, cleaned and refilled the pool 2 years ago. Since then, I have struggled to keep chlorine in the pool. This year has been especially hard to keep chlorine in the pool. I had an algae problem recently as well, which would abate with high dose shock, but come back in a week or so.

I started the slam process last week and the pool cleaned right up within days. Water is crystal clear. I replaced the filters a few days into the process as well. My morning tests usually come back with less than 1 ppm of CC. However, for the OCLT, I'm dropping to between 1 and 3 ppm of FC when I started the night around 16 ppm of FC. I have been losing 15 ppm of FC nightly for the past 9 days!

Here are my current numbers using Taylor k-2006 FAS-DPD:
FC: 0
CC: 0
Ph: 7.6 (started process at 7.2)
TA: 100
CH: 580
CYA: 40

 

[wireform]

I would bet you have an algae problem with the underside of those boulders and or the water feature. You also need to brush every nook and cranny as that's where algae hides. look in the skimmer behind the weir door .....