We are on week 3 after opening our new vinyl pool. In order to keep in good standing on the warranty from the pool builder, I've been having them test my water weekly and following their recommendations.
Week 1 - Added 2 gallons muriatic acid, 2.5 pounds of shock powder
Week 2 - 2 more gallons acid, 2.5 lb. shock, 5 lb. stabilizer (sock method)
Now at Week 3 they are recommending I shock it, despite the fact that the numbers all fall within their recommended ranges.
FC = 3.68 (2-4 recommended, seems low, but that may be because we have an ozone generator)
TC = 3.96
CC = 0.28
pH = 7.4
CH = 238
TA = 108 (week 1 was 220, week 2 was 170)
CYA = 44
It all looks pretty good to me. I have been using trichlor pucks in the chlorinator. It ran out for about half a day and I replenished it, so FC may go up a little from where it is right now. Right now the auto chlorinator is on the highest setting. Then again, I had the solar cover on for a few days, so this weekend we may burn off some FC in the sun. Looking at highs in the upper 90's and our pool is in full sun until late evening.
So do I really need to put shock in it again this week? They advocate weekly shocking, but intuitively this seems unnecessary unless you're having trouble keeping the numbers in line.