Locked dirt on bottom of pool


Jul 16, 2019
Houston, Texas
I bought a house with a pool in good condition a month and a half back. 14,000 gallons, concrete, and waterfall. I came across this site about a two weeks into ownership and started a purely liquid chlorine sanitation with good results so far.

Prior to switching, I had a run in with mustard algae. I left for a vacation to Alaska and gave my wife instructions. She added additional pucks to the chlorinator when needed, and ran the vaccuum twice a week. I failed to tell her to watch the pump for clogs as the vacuum was suction side directly into the pump and filter. The pump clogged up and couldn’t run enough water through the chlorinator. The chlorine was at zero when I got home. Add to it some impressive rain storms and we had a major mustard algae outbreak.

To clear it up (before I found this site), put in a lot of cal hypo. Enough for 30 ppm according to the chlorine calculator. And brushed like crazy. After 3 treatments the algae was gone. I washed out the cartridge filter. The light yellow stains disappeared in a couple weeks with correct chlorine levels.

After using the cal hypo, I noticed brown spots that looked like dirt on the bottom of the pool. Somewhat evenly placed across the area I dumped in the cal hypo. I did not brush the cal hypo in on two treatments. I just poured and went to bed.

The stains do not respond to brushing. And are not slick like algae. From what I’ve read it seems like the calcium compounds in the cal hypo may have locked in some dirt with scale buildup.

The pool is sparkling clear. I only use liquid now. It’s just these Dang brown spots. And I can’t get rid of them with liquid shock and brushing. Not sure what to do.

Anyone have any thoughts on what this is? And what could get rid of it? I haven’t ruled out metals either as Leslie’s didn’t have the test available and my k-2006 doesn’t have that test. I’m sure there is some copper content in our city water with old copper pipes.

My chemical levels:

FC: 4 ppm
CC: less than .5 ppm
TA: 70
Calcium: 270
Cya: 45 ppm

I am a nut with the LSI so I’m usually right around 0.


Mod Squad
TFP Expert
LifeTime Supporter
Jul 10, 2009
Houston, Texas
If it is calcium deposits anchoring dirt to the plaster, keeping the pH around 7.0 to 7.2 can help put it back into solution over time. If the stains/deposits are not in the deep end you may try using a length of pvc pipe to deliver a small dose of muriatic acid directly to the stain, then brush and see if it lifts.
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