Started battling algae April 30th, but not consistently until the last 6 days. I have a 36,000 gal pool (in Alabama) and use a TF-100 test kit. I no longer have the numbers when I started this battle, but I recall all the numbers being in an acceptable range to start the SLAM process. When I began, the vinyl liner was covered with green algae. I will point out that I watched it spread from a spot in the shallow end that never gets sunlight. This "spot" has been with me since last summer and remained all winter long. (I never did get a hold of it last summer). Imagine about a 2 inch wide green spot that follows the floor of the pool against the wall that never gets sunlight. Anyway, I point that out because this seems to be the origin of the problem and this spot is impervious to any attempts to brush it away.
So, I start the SLAM, and I made multiple mistakes, so I am going to fast forward until I got a reorder on my FC test kit and got serious about brushing and really checking things. This started 6 days ago. Importantly, after reading a lot and looking at images on the web (and based on my experience of no algae problems for years and then suddenly recurring algae problems as soon as I drop the ball in testing), I am (now) convinced that I have a mustard algae problem. However, when I first started the SLAM I tested my CYA (45) and sought to shock the pool at a level that I believed would be higher than needed (I chose FC=18). The first few days I did not check it enough, because I kept seeing readings that would go below shock value (FC=16). After two days of lazily checking, I started checking constantly. Now, here is what may be an important variable. Once I thought that I had mustard algae, I initially interpreted the "mustard shock" value as the value that I had to maintain. (I now know that you do this for 24 hours after passing the tests). So, for about 3-4 days I had the FC in our pool at around 30. One time the reading was FC=42.
Yesterday,I finally saw the proper procedure for dealing with mustard algae, but I figured that since the FC was so high I would go ahead and hurl all of the pool equipment into the chlorinated water. So this is what I did this morning. I let everything marinate in FC=42 water for two hours and then pulled it all out. Afterwards, I vacuumed out the pool and backwashed it. (Note when I did this there were mud-loooking stains on the walls in spots, and I was mostly vacuuming what looked like sand). Because so much water was lost, I filled it back up with tap water. My mindset was that I had seriously over chlorinated my pool and that I was going to damage the liner. I thought that there was no possible way to go from FC=42 to a reading below shock value (FC=16) any time soon. I decided to conserve my powder (Edit: testing powder--I've not put anything but bleach in the pool) and chemicals (edit--testing solution) and await tonight's reading. Honestly, I was thinking along the lines of passing the overnight chlorine loss test. After all, I've dumped untold gallons of bleach in the pool since I started.
Below is tonight's situation:
CYA: 45 (didn't test again, but this is what it was when I started)
Impervious stain is still there, unaffected. I've brushed it a thousand times. Last year I remember trying to rub Vitamin C on it for some reason. Water is crystal clear. I spent a ton of time today brushing every square inch of the pool. I will note that when I brushed a crevice below the steps I did see a big plume of (what I think is) algae come out of it.
1. Why is my TA now so low? Should I do anything about it or is this just a false reading due to the SLAM?
2. If I one day pass the OCLT do I still need to do the 24-hour "mustard shock," given that I have erroneously been doing this for days now?
3. What the heck is this stain? I promise you it looks just exactly like algae. Imagine a line on the floor of the pool that hugs the wall. It starts out algae green/yellow and then gradually turns into just a blur.
4. I read somewhere on here that dead algae is "colorless." I am getting up every morning and seeing what looks to be lines of sand in the pool. It is mud brown.
5. Can throwing pool equipment into the the water cause FC to drop 30 points, or is this more likely indicative of a continuing algae problem more generally?
I apologize that I went off on the SLAM half-cocked and am not a good representative of the process being espoused here. I guess I got cocky and a little lazy. However, I would appreciate any advice here. My Plan: Keep shocking until I pass the OCLT and ignore the fact that I will still have the impervious stain in the shallow end. Then, I will keep the FC at double the minimum rate until the stain (hopefully) disappears in a few weeks. Does this sound reasonable?