All levels are correct, but pool won't hold chlorine

[danielpatrick]

You are not raising FC to 10 and doing a SLAM Process.
SLAM level for your CYA of 40 is FC of 16 - as indicated in the charts in post #11.
You are merely raising your FC to 10 to start your Overnight Chlorine Loss Test.
Unfortunately, you will be unable to accurately test the starting and ending FC of the OCLT without the Taylor K-1515 testkit (R-0870 and R-0871 reagents).

Don't confuse the word "shock" with SLAM - they are not interchangeable. SLAM Process

Thank you for explaining. I'm not confusing shock with SLAM, but maybe that's for someone else scanning this thread in the future.
10ppm came from the PoolMath app when I toggled SLAM. I'm going to start with the SLAM level from the forum (16ppm), even though I don't have a kit that can give me the results I need (that's on order now). It's still massively more than the anemic amount I was dosing with before.

 

[mknauss]

The app bases the SLAM level on the last saved CYA level. Yours is likely shown as 0.

 

[PoolStored]

I'm going to start with the SLAM level from the forum (16ppm), even though I don't have a kit that can give me the results I need (that's on order now).

Please don't. Add 5ppm per day until the test kit gets there. Then start the SLAM. We don't want to damage pool or equipment.

 

[Bperry]

If you’re still having trouble holding any chlorine at all, you may try and add 5-10ppm and check it again 30 minutes later to rule out ammonia. That may have been where the “10” came from as well. It won’t matter the exact amount of chlorine but if it holds any after 30 minutes you should be clear of ammonia.

 

[danielpatrick]

Long story short: Unidentified mustard algae was consuming huge amounts of chlorine. SLAM knocked it back, chlorine levels finally are being held. Will proceed with mustard algae shocking level to finally knock it out.

The issue is (probably) resolved, or at least definitively identified. After raising the chlorine to 16ppm on Wednesday afternoon, the pool consumed 15.5ppm overnight. Brushed the pool and washed the filters (after a couple of hours). Redosed to 16ppm Thursday AM and this time it stuck. Still didn't have test kit, but chlorine was greater than 5ppm so didn't dose. Brushed the pool and washed the filters every morning. Pool was holding chlorine (not consuming all of what I was adding) and I didn't have to add anything for the past 3 days.

The main culprit? Some dark staining in the deep end that I had presumed to be caused by low calcium levels was actually algae instead. Daily brushing with my stiff nylon brush wasn't removing anything / showing any telltale plume when disturbed. I biffed on one of the passes and accidentally scraped with the top (ferrule?) of the brush exposing a more normal color behind it. So I brought out stainless steel brush and saw the algae cloud and reduced darkness. Over 3 days I brushed multiple times (especially in the trouble areas) with decreasing clouds and a corresponding amount of algae in the filter.

The kit finally came in today. I ran a full battery of tests now that the chlorine has dropped to 5ppm.

FC 4.5
CC 0.5
TC 5.0
PH 7.8 (to be expected with the amount of chlorine I added)
CYA 40
TA 90
CH 225

Based on the behavior of the algae (was only settling on the bottom of the pool not the walls, only grew in the shaded portions, mustard yellow when brushed) it's very likely mustard algae.

The plan is to bring the pool back to SLAM (7.2 PH, 16ppm Cl) and then raise to mustard algae shock level (23.8 ppm Cl for 24 hours as per this). Will wash everything.

I wonder if I can leave the pump driver robot in during this step? I'm sure some elements of the algae are present in the hose / structure of the robot.

Thanks to everyone that contributed with their suggestions / support.

 

[Newdude]

I wonder if I can leave the pump driver robot in during this step? I'm sure some elements of the algae are present in the hose / structure of the robot.

Sure can. The HOCL component of the FC is the 'harsh' part and can be over 4X stronger in tap water than regular SLAM. So going to mustard algae slam, or 1.5X over regular SLAM is still not as harsh as tap water, yet you'd soak it in hose water without giving it a seconf thought. Lol.