read my reply to CarlD in this thread about my observatons on CYA and SWGs
http://www.poolforum.com/pf2/showthread.php?t=2785

Once you get the CYA in there you probably won't need to shock and will mostg likely need to turn the output on your cell down!

Get your ALK up and then get your pH down afterwards. Keep monitoring the pH and try to keep it as close to 7.2 as possible when you ajust it down. It will rise soon enough on it's own! Try not to let it get above 7.8 (7.6 is a better number to shoot for as your upper limit)

To raise your CYA from 0 to 60 you will need 9.5 lbs for 20000 gal. 60 is the lowest I would recommend you run it with! I have noticed problems with CC in my pool and my customers pools when the CYA is 50 or below with a SWG...don't know why but I've seen it time and again. I know it goes against the accepted info on the forum but I have been seeing this time and again. I suspect it is because of the way the chlorine is introduced into the pool with a SWG. It really isn't the same as dosing manually.

We have a demo SWG cell at work that is a lucite tube with 2 electrode plates in it and a battery for a power source. We put salt water in it and 5 drops of OTO to show customers how the cell generates chlorine. When we press the on button the water in the area around the electrodes immediately turns deep orange to brown showing extremely high chlorine concentratons right around the plates and a much lighter yellow color throughout the water (maybe about 1-2 ppm). this shows how the water is being 'supershocked' right around the electrodes. If we keep the unit on the entire water column in the tube will turn brown in a matter of seconds. I cannot think of any other way of introducing chlorine into a pool that is like this. Perhaps this is why the high CYA levels and lower chlorine levels work with a SWG.