Is this dead algae?


New member
Jun 23, 2018
Caro mi
Hello. I have read every article on here and many other threads looking for a solution to what I think is algae. 3 or 4 summers ago I had mustard algae. It was text book mustard algae. I let the chlorine levels go to nothing for a day or two and when I took the solar cover off I had an omg moment. It was yellow, mostly on the shady side, but everywhere in the pool including the walls. I shocked and shocked the pool, I used algaecide and after a few days the pool was back to "normal". I then started noticing what I thought was sand in the bottom of my pool. My husband was restoring an old truck and was sandblasting a lot that summer. I thought it was sand from him sandblasting. It has been back every summer since and I fight it all summer long. I found this forum last year and figured out my part of my problem was my CYA was way too high, over 100. I now have it back down to 30 ppm and have switched to liquid chlorine instead of the trichlor pucks. I still have the sand looking substance on the bottom of my pool, always in the same spots in low spots/divots in the bottom. It's not sand, it poofs away when you try to pick it up. I followed the articles on here and did a SLAM two weeks ago. After I passed the criteria to complete the SLAM, I then did the extra high mustard level for 24 hours (I actually did slightly higher FC levels than the chart recommend for good measure). I've been keeping the FC on the higher end of where it's recommended and I seem to be going through a lot of chlorine. I am using the trichlor pucks in addition to the liquid chlorine because we have had a lot of rain here in MI this summer and I want to raise my CYA a little. I'm attaching a pic of the sandy like substance, to see if you think it is dead algae or something else.

I put a gallon of 12.5% chlorine in on Friday night as I was gone overnight Saturday and wanted to ensure I had enough chlorine to last in the event my family forgot to add it while I was gone. My husband did add a half gallon last night. When we removed the solar cover at noon today, I was so excited to see a completely clear bottom with no sandy substance...I thought maybe it was finally gone. At 5:00 today I looked to find sandy stuff all over again. I am so frustrated. I don't know what this is or why I can't get rid of it. The water is crystal clear.

I have a 20,000 gallon pool with sand filter and a hayward heater and here are my test numbers I just took:

FC 5
CC .5
CYA 30
PH 7.5
Alk 100
CH 180

I appreciate any advice!



New member
Jun 23, 2018
Caro mi
Thanks for the fast reply. I have read those articles, but in the SLAM article it says the SLAM is complete when the CC is .5 or less, I pass the OCLT test and the water is clear. If I pass those things and am still seeing dead algae, does that mean I need to keep slamming until I no longer see the dead algae? Is that what you think I have is dead algae? Also, when SLAMming, I either brushed or vacuumed the pool each time I added chlorine, so several times a day. If I brush the pool, will the dead algae oxidize or do I have to actually vacuum it out each time I see it? Or is just brushing it up enough? So, I'm thinking I need to do another SLAM, right? This time, do I do it at regular SLAM levels until I no longer see any dead algae, then the mustard levels for 24 hours or just the regular SLAM levels until I no longer see dead algae?

Thank you very much!


Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
Laughlin, NV
If you have dead algae in the water, the water is not clear. Brush it and the water is cloudy.
You can vac it up but it is typically very hard to do so. Brushing it up and into the water gets it to the filter.
Can you fill out your signature? Helps us help you.

Just do your normal SLAM levels until you pass all three criteria. If you are sure you have mustard algae (very rare), then you raise FC to the higher level for 24 hours and then maintain the higher MA target FC for several weeks.