SLAMMING, MA, and verifying CYA levels


Bronze Supporter
Jun 25, 2020
Pittsburgh, PA
1) Verifying CYA:
I recently found a CYA chart here that shows how much FC is lost in full sun given various CYA levels. I know that testing CYA is a bit of an art, so I was able to verify my CYA reading with the chart. I read 30 for CYA, and the FC loss during the sunny, clear day last week confirmed the reading was correct. I added the recommended amount of CYA (cautiously...because I don't want to risk going too high), and got the CYA to the "ideal" reading of 40. I was able to again confirm that reading with the FC loss chart on Saturday (which was also a clear, sunny day). The math works, so I guess it is accurate enough. So, if you are questioning your CYA reading, verify it against the chart posted by TreeFiter using the math Chem Geek provided.

Last year I suspected that I may have had a lurking mustard algae problem. I wasn't sure, but there was always a yellowish tint to the white fiberglass walls, especially in the shadowy areas of the pool. Brushes didn't touch it, and Scotch Brite pads got some off, but not all. There was also constant puffs of dead algae clouds in the corners when I would brush. I opened the pool this year to slightly cloudy water, and began to SLAM. For the last 3 days I have maintained Mustard Algae SLAM levels found in this post. I know that's not recommended. The pool is still cold and no one is swimming in it anytime soon, so I gave it a shot. With skimmer socks, constant vacuuming, and daily brushing, the water cleared, and the water and walls are now GLEAMING white. No yellow, no staining, and the pool looks SO clean.

Here's the strange thing:
All SLAM I passed the OCLT with 0 FC loss and 0 CC ... up until last night. Last night I saw an OCLT loss of 8! Either my initial FC test was wrong, or something else. It is almost as if something "broke loose" last night... the water cleared, the walls turned gleaming white, and the large FC loss happened. Here's hoping tonight's FC loss is 0 (but it is raining hard here today, so might be wishful thinking)!

Texas Splash

Mod Squad
TFP Expert
LifeTime Supporter
Well, the 2.5 OCLT can't be ignored, so something is still lurking in there. It may be the smaller details. Do you have a main drain? If so, you may need to pop the cover off once the water warms to see if there's junk under the lid. Also look closely around the skimmer opening, weir door, and even under the skimmer lid itself.
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