I can't let this go - one app shows you at 79, another shows you at 50. So go to 20 or 31 for SLAM? Did you just waste $30-40 by putting too little in? Nobody can tell you.....
I will take it CYA of 50. My understanding is:
Keep pool water clear is actually a very straight forward job, threw a bunch of disinfectant (in this case it's the chlorine, or TC/FC) into the pool, it will stop the algae growth, pure and simple.
But, the trouble is how much! Too much will bleach your swimming pants, too little will not kill the bugs.
Seemingly simple job will frustrated enough to puzzle the PhD student, firstly because it's entangled with the pH. The potency of chlorine is depended upon the acidity or basicity of the pool water.
Secondly, the sun light will decompose the chlorine, therefore, this is how the famous cyanuric acid (CYA), the "chelation agent" come into the picture.
Cyanuric acid / Formula
C₃H₃N₃O₃
CYA has 3 nitrogen atoms, which can form complexes with chloride atoms to "Shield" them away from UV deposition, so it also reduces the effectiveness of chlorine. Put it simply -- it bonds to the chlorine to reduce the amount of FC (free chlorine) present in the pool water.
Okay, by saying so much "mumbo-jumbo" won't help much unless you realize that: high pH is not good and you need some CYA, but high CYA will also "lock up" your TC to form CC, which also decrease the effectiveness of your disinfectant. So there must a balance of the whole thing, that's the point. (Well, there still other stuff like alkalinity, Fe, Cu, P to concerned with but they are minor things). If you managed to get your FC between 1 ~ 3 ppm, you are all set!!
Now, let's come back to the question of how important the CYA really is? Well, if you are in somewhere not so much sun, you can use less or forget about it all together! For instance, if I have an in-door pool, I don't need it, because it tight up my FC which force me to dump more bleach to kill the bacteria. So, CYA can be as little as 0 - 20. Well, if I am in Texas or California or Florida, I can use more of that stuff. Well, you can put 80 to 120 ppm. If you ask me if CYA is that critical, my answer would be: probably not! If I don't use it or use less ----- it probably means every day I dump more bleach into the water to compensate the UV loss of chlorine! That only means being a fool that I waste some money to it, that's all. You will figure out pretty soon go up a little with CYA will save you money!!!! That's all there is to it, my friends! The experts even don't agree from each other on how much CYA you really need, so don't put too much, because it eventually will lock up your chlorine that no matter how much BLEACH you pour into the pool, it still won't stop your algae boom, then you are in a big problem!!!! So using too much of it is really a bad thing from this point of view. Furthermore, the chlorine tablet has tons of that stuff mixed in, that's why a lot of people can easily get too much of it into their pool without knowing it. It's a pain on the Rear, because in order to get rid of it you have drain some water and replace some fresh water, which is something I really reluctant to do.
All on all, regardless how inaccurate the numbers are, showing variety of the numbers, varying from 50, 70 or even 100 of CYA, well the upper bound I heard is 120, will that cause a deadly error, messing up my pool? I really don't think so.
Being bold and having some fun my friends!
Lastly, my very "accurate" numbers pulled just this morning. Looking forward to your comments!
www.troublefreepool.com
with xtra stuff (pool store potions 
)
