JasonLion said:
CYA does bind to some/most of the chlorine, but the resulting chemicals leave the water completely transparent. There are many things that can cause cloudy water, but simply having CYA in the water won't do it. Adding CYA will change the PH and if you pour it into the skimmer it can disturb the filter, either of which could cause cloudiness in some situations.
I thought of two amendments to the above statement. First, CYA does not cause cloudiness in and of it's self
in the visible spectrum, but it does absorb light in some bands not visible by humans. The science is a little sketchy, but it appears that some of this absorption of light not visible to people may indeed be related to the reduction of chlorine consumption by sunlight at higher CYA levels.
Second, adding CYA changes some other chemical balances, besides PH. Significantly, it reduces the proportion of chlorine that is in the active disinfecting form. That can have substantial indirect effects on the water chemistry, though mostly over days and not over seconds.
"Chlorine lock" is a myth, or a place holder if you will, for a process that was not well understood until fairly recently. High TDS and CYA levels tend to be associated with pools where there are nominally "acceptable" chlorine levels and yet algae develops. Because the relationship between CYA and the concentration of the active disinfecting form of chlorine was not understood, a myth developed to try and explain what was happening in a memorable way. The reasoning in the myth is flawed, even though the effect is real.
When the FC level is held constant and the CYA level is raised the pool will first become cloudy with early, yet partially controlled, algae growth. As the CYA level continues to rise a break point is reached where a full fledged algae bloom develops. Since no one knew that the FC level had to be raised when CYA was added to the pool, the cloudiness was described as being caused by the CYA. The cloudiness is indeed caused by the CYA, but in a fairly indirect way. Increased CYA results in reduced active chlorine levels, which allows algae growth, which causes clouding.
Interestingly, the cloudy stage before a full algae bloom is greatly extended at high CYA levels. At high CYA levels and "conventional" chlorine levels the active chlorine level is low and algae can develop. However, as the algae is killed by chlorine additional active chlorine is released by the CYA and additional algae is killed. At appropriate CYA levels the rate at which the algae grows and the rate at which active chlorine is released is more or less in balance and so the algae does not get fully going for some time, nor is it completely killed off, until the chlorine reserve bound to CYA is used up. This behavior is essentially invisible at lower chlorine levels, so cloudy water caused by algae, rather than fully green water, was not an effect that people where familiar with other than at high CYA levels. In many circles it was not understood that the cloudiness was actually caused by dead algae, and it was assumed that it was caused in some more direct way by CYA.
The pool industry is full of myths like this one. The chemistry of swimming pools is fairly complex and isn't really well studied. A great number of stories told by old pool hands to explain the things they have seen over their lives. Many of these stories have become accepted as truth despite the fact that many of them have no scientific basis. In several cases the stories promote practices that simply don't work at all. In other cases the stories give good advice, but the advice works for reasons other than the reasons given in the story.
The advice that CYA should be kept under 100 is good advice but the implied reasoning behind it is flawed. It is completely possible to maintain crystal clear water at very high CYA level. That is a simple observable fact that has been duplicated by thousands of different people. So, observably, the presence of high levels of CYA alone does not cause cloudy water (in the normal visible range). However, high CYA levels do indeed frequently result in cloudy water in practice, through indirect effects, something that is also well documented.
