We have a 10 year old concrete/plaster pool with a Pentair chlorine generator and automatic cover. For the first 9 years, it was virtually maintenance free. (Acid once a week was about it.)
Because it was covered 90% of the time, I didn’t use CYA but monitored the chlorine carefully.
This year was different. When I opened the pool in the spring, I noticed dark discoloration on the metal pool light trim. It wiped off easily so I pulled the lights and discovered a bunch of nasty blackish algae behind them.
I think it is important to note that at no time was the pool anything other than crystal clear.
I shocked the pool and everything seemed ok for about 2 weeks. I also noticed the pool light trim had deteriorated so replaced them.
Due to grandchildren getting older and learning how to swim, I decided it was time to start using CYA because the pool would be open more. I keep the concentration low (30-40 ppm) even though there is a chlorine generate because the pool is still covered the majority of the time.
About 2 weeks later, I noticed some yellowish algae-like stuff on the surface of the metal light trim.
I had also found troublefreepool.com so followed the SLAM protocol.
Everything was fine for another 2 weeks or so when the algae-like stuff appeared again. I pulled the lights and discovered some algae living in the light conduits. I sealed the conduits with a combination of plug and pool putty. I SLAMed again and treated at mustard level for 24 hours.
After that, I maintained a chlorine level of 8-10 ppm, well above the mustard inhibition level for my CYA concentration. It seems like every couple weeks I get the algae-like stuff around the lights and have to SLAM again.
I was in the pool yesterday, and noticed the algae-like stuff is back on one light. I ran an OCLT overnight and there was no loss of chlorine (10 ppm).
At this point, I’m at a loss. The situation is manageable but I don’t understand it.
1) How can I still get algae growing at this chlorine level?
2) Why only around the lights? (Never, ever on the pool itself.)
3) Can algae penetrate the pool light gasket and act as a chlorine-resistant reservoir?
4) Is it possible that this is NOT algae but something else entirely?
5) I’ve had to add almost no acid this year. For the previous 9 years, I had to add it weekly.
6) I've tried to get the TA down to <80 ppm but it has proved difficult. The pool pH almost never goes above 7.4 even though I use an aerator
Any insight would be appreciated.
FC 10.0
pH 7.3
TA 90
CH 475
CYA 30-40
SALT 3500
Because it was covered 90% of the time, I didn’t use CYA but monitored the chlorine carefully.
This year was different. When I opened the pool in the spring, I noticed dark discoloration on the metal pool light trim. It wiped off easily so I pulled the lights and discovered a bunch of nasty blackish algae behind them.
I think it is important to note that at no time was the pool anything other than crystal clear.
I shocked the pool and everything seemed ok for about 2 weeks. I also noticed the pool light trim had deteriorated so replaced them.
Due to grandchildren getting older and learning how to swim, I decided it was time to start using CYA because the pool would be open more. I keep the concentration low (30-40 ppm) even though there is a chlorine generate because the pool is still covered the majority of the time.
About 2 weeks later, I noticed some yellowish algae-like stuff on the surface of the metal light trim.
I had also found troublefreepool.com so followed the SLAM protocol.
Everything was fine for another 2 weeks or so when the algae-like stuff appeared again. I pulled the lights and discovered some algae living in the light conduits. I sealed the conduits with a combination of plug and pool putty. I SLAMed again and treated at mustard level for 24 hours.
After that, I maintained a chlorine level of 8-10 ppm, well above the mustard inhibition level for my CYA concentration. It seems like every couple weeks I get the algae-like stuff around the lights and have to SLAM again.
I was in the pool yesterday, and noticed the algae-like stuff is back on one light. I ran an OCLT overnight and there was no loss of chlorine (10 ppm).
At this point, I’m at a loss. The situation is manageable but I don’t understand it.
1) How can I still get algae growing at this chlorine level?
2) Why only around the lights? (Never, ever on the pool itself.)
3) Can algae penetrate the pool light gasket and act as a chlorine-resistant reservoir?
4) Is it possible that this is NOT algae but something else entirely?
5) I’ve had to add almost no acid this year. For the previous 9 years, I had to add it weekly.
6) I've tried to get the TA down to <80 ppm but it has proved difficult. The pool pH almost never goes above 7.4 even though I use an aerator
Any insight would be appreciated.
FC 10.0
pH 7.3
TA 90
CH 475
CYA 30-40
SALT 3500
