Brownish debris in bottom of pool, but only after running pump

[Koomazaz]

Hi guys,

I'm a relatively new pool owner (two seasons now), and I have two potentially-related questions.

Background info:
Aquasport 52 (17' x 32', oval, ~14k gallons)
Flat bottom, no deep end
Two years old, set up last April
I live in north Alabama, pool is in partial or full sun all day
Trees are on the lot next to my house, but nothing is overhanging the pool itself
Sand filter, single speed pump, on a timer
Skimmer and water return are near one of the "points" of the oval
Pump turns on for 6 hours each night at sunset
I use liquid chlorine, 10%, from Home Depot or Walmart
I test FC nightly and everything else about each week
I have a TF-100 test kit

First issue:
I *think* my CYA is around 60 according to the CYA test, hoping that it would reduce the amount of chlorine I have to add each night. I've been recently targeting 9 FC, and each evening when I test, we're hovering around 4, so losing about 5 a day. That seems high.
Water is crystal clear. I vacuum regularly and skim as needed. I know that I might need to SLAM, but I wanted to mention my second issue before jumping to that conclusion.

Second issue:
All throughout the day, the pump is off, and the pool looks great. Each night, I add chlorine before I go to bed and the filter runs for 6 hours. In the morning, there is brownish debris (very fine, floats away with a water current) that is collected on the floor, on the half of the pool nearest the water return. It gathers in the divots and imperfections in the pool bottom. It vacuums up, no problem, and the pool looks great for the rest of the day.
I would assume algae and I need to SLAM, but here's the weird part.
I lost some water due to an accidental disconnected hose, so I wasn't able to run the pump two nights ago. Yesterday morning, after NOT running the pump throughout the night, the floor of the pool was pristine. No brown debris at all. I added water so I could run the pump again.
Last night (about 48 hours after last adding chlorine and running the pump, essentially skipping one day of maintenance), my FC was still at 3.5 ppm. I would have expected it to be around 1 ppm, since on a usual day, I lose about 4 per day.
I added chlorine up to 8 FC , and ran the pump for 6 hours.
This morning, there is brown debris again.

So, to summarize, on a normal day I seem to be losing more FC each day than I would expect. But on the day I skip running the pump or adding chlorine, I lose less FC than expected and there is no brown debris on the floor. After adding chlorine and running the pump, I see brown debris again.

I expect that the first response will be to SLAM, which I will be starting tonight. But I'm not sure why I would not see as much or more debris on the day where I skipping running the pump. That part doesn't make sense to me.

Any ideas, thoughts, suggestions? I will be SLAMMING tonight.

I'll add better photos of the debris after it stops raining.

Thanks!

 

[Texas Splash]

Welcome to TFP! Before going right into a SLAM Process, I would be inclined to perform an Overnight Chlorine Loss Test. The OCLT should really confirm if you are experiencing an unusual FC loss due to organics. If so, it's possible algae is traveling into the pluming or filter and blowing back out when you run the system. If you pass the OCLT, you might consider doing a deep clean of the sand.

 

[Koomazaz]

Welcome to TFP! Before going right into a SLAM Process, I would be inclined to perform an Overnight Chlorine Loss Test. The OCLT should really confirm if you are experiencing an unusual FC loss due to organics. If so, it's possible algae is traveling into the pluming or filter and blowing back out when you run the system. If you pass the OCLT, you might consider doing a deep clean of the sand.

Hello! Thanks for the welcome and the information!

I did an OCLT last night, following a full spectrum of tests.

FC: 7
pH: 7.8
TA: 70
CH: 125
CYA: 30

As you can see, the CYA was much lower than I thought. We've lost some water recently and refilled, so I've obviously lost some. Do you normally recommend setting the CYA tube on a light or dark surface?

So, I've been bringing FC up to 9 on a nightly basis, but last night, I brought it up to 7 instead.

This morning, I measured 6.5, so I think we passed the OCLT.
I'll plan to do a deep clean of the sand filter tomorrow.

 

[Texas Splash]

This morning, I measured 6.5, so I think we passed the OCLT.

That's great! Below are my notes about the CYA test if it helps.

CYA Testing:
Proper lighting is critical for the CYA test, so you want to test for CYA outside on a bright sunny day. Use the mixing bottle to gently mix the required amounts of pool water and R-0013 reagent, let sit for 30 seconds, then gently mix again. Recommend standing outside with your back to the sun and the view tube in the shade of your body at waist level. Then, begin squirting the mixed solution into the skinny tube. Watch the black dot until it completely disappears. Once it disappears, record the CYA reading. To help the eyes and prevent staring at the dot, some people find it better to pour & view in stages. Pour some solution into the viewing tube, look away, then look back again for the dot. Repeat as necessary until you feel the dot is gone. After the first CYA test, you can pour the mixed solution from the skinny view tube back to the mixing bottle, gently shake, and do the same test a second, third, or fourth time to instill consistency in your technique, become more comfortable with the testing, and validate your own CYA reading. Finally, if you still doubt your own reading, have a friend do the test with you and compare results.

 

[Texas Splash]

I'll plan to do a deep clean of the sand filter tomorrow.

Here's the link in case you need it.

Deep Cleaning a Sand Filter

This post will walk you through the anatomy of a sand filter, and then provide a video at the end for how to deep clean your sand filter. The filter used in the photos and videos is a Hayward S180T top mount sand filter. The images are of a similar filter. Obviously not all filters are...

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