As I reported in this post, I started up the solar system in my pool to get ready for the start of the swim season. Prior to this, the chlorine usage was very low at around 1 ppm FC or less per week. So last Saturday, 3/28, I added some Cyanuric Acid in the skimmer through an old T-shirt (so I could see if it was dissolving). The skimmer has a bypass so with a lot of CYA clogging it, most of the flow was in the bypass, but some went through the CYA as well. I checked the next day and the CYA was all gone so with this method and the pump set up higher (48 GPM) CYA seems to dissolve rather quickly. The CYA also had a measured increase of 20 ppm out of the 30 ppm I expected so it seemed to mostly dissolve. I also added one box of 20 Mule Team Borax to raise the pH (and to add some Borates, though I'm getting Boric Acid later to get it up to 50 ppm). So far, so good.
I had raised the FC level to 3-4 ppm on Saturday and figured it should be good for the week given the cooler temps and not opening up the opaque electric safety cover (i.e. no sunlight). Wrong! I opened up the cover yesterday (Saturday, 4/4) to find the water cloudy! I could still mostly see the main drain, but the water was most definitely cloudy, not just dull looking. I measured the FC and it was zero and the CC was 1.6 ppm. I had the beginnings of an algae bloom. Yikes! I immediately added some chlorinating liquid I had on hand, but knew it would not be enough. I also measured the CYA level and it was 0 (or at least a lot less than 20 ppm)!
So, today I went out and got some more chlorinating liquid as well as an ammonia test kit since the pool seemed to smell like ammonia and monochloramine when I added the chlorine to it. I tested it today and the FC was still zero but the CC was down to 0.8 ppm and the ammonia reading was around 2 ppm. If I assume that the warmer water got bacteria and algae to use up the chlorine faster, then perhaps the 3 ppm FC lasted 3-4 days which meant I had 3-4 days with zero chlorine. Apparently, that was long enough for bacteria to have a happy meal of CYA converting it to ammonia. The CC of 1.6 ppm is equivalent to 0.3 ppm ammonia and the 2 ppm ammonia I measured means a total of around 2.3 ppm ammonia. That corresponds to around 8 ppm CYA. I'll bet that a lot of the ammonia got eaten up by the algae which is what is making the water cloudy. I knew that water could turn cloudy from algae growth after a couple of days or so, but had no idea that the bacteria could grow and consume the CYA so quickly. Some people have reported fairly rapid conversion, but just a few days is pretty darn fast.
So, now I've got a lot more chorinating liquid in the pool and will retest the ammonia, FC and CC in an hour to see where I stand. Does anyone know if the ammonia test will get interference (accidentally measure) monochloramine? I don't know how the ammonia test works. With my separate CC measurement which is not interfered by ammonia, I can adjust for any interference in the ammonia test -- I just don't know if I need to do that. It's pretty approximate anyway since the ammonia test color comparison isn't precise (i.e. it's not a FAS-DPD, TA or CH kind of drop-test).
The lesson from this is simple. When starting up a pool and warming up the water, assume that the chlorine usage may go up considerably. Test frequently, which in my case (with the opaque pool cover) would have been at least a mid-week test so that the chlorine would have never gone to zero. During the swim season, I always test twice a week and add chlorine about twice a week which works fine for me and is quite predictable. I could just kick myself for not doing it during startup.
I'll keep you posted (in this thread) on my progress as I expect to clear this up quickly and will see if the CYA drop corresponds to the chlorine consumption from measured CC, ammonia and amount of added FC needed to eliminate the algae. I purchased some additional CYA, but will add that after I get this under control (might as well kill this stuff and oxidize chloramines quickly -- I don't care about nitrogen trichloride since it's an outdoor pool I can air out as needed).