Your chlorine is too low, so you'll need to add bleach before & after each use. Your PF is 5, so 1 gallon of 6% household bleach will add (5 x 0.5 lbs Cl2/gal 6%) about 2.5 ppm of chlorine to your pool.

Add 1/2 gallon 15 minutes before swimmers enter, and after they leave. Also if it's sunny, add 1/4 gallon every hour during use. If you can add via the skimmer, that would be best -- just keep swimmers away from the returns for 15 minutes after you add, so their swimsuits don't get 'white-spotted'.

BUT . . . you need to get some CYA in that pool. Dichlor is the easiest way. I've copied some info on that, below.

Other than that, you need to keep adding large doses of bleach at night, till you can hold chlorine levels overnight. If there are any tabs in the feeder, it would be best to remove them. Bleach in the skimmers is not likely to cause a problem, but it's better to be safe.

Also, you should detergent clean your filter when you get a chance. With those System3 beasts, you'll need quite a bit of TSP and a BIG garbage can, but it needs to be done. Here are the instructions: http://www.poolforum.com/pf2/showthread.php?18044 . Remember, you also need to read the S-3 manual: http://www.pentairpool.com/pdfs/System3SMOM.pdf . And, you'll need a helper to lift those cartridges in and out of place without damaging anything!

If you want to do a chlorine demand test, to see how much more bleach it's likely to take, here are instructions for that:
http://www.poolforum.com/pf2/showthread.php?16973

Ben

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+ If you need stabilizer, and have access to a Sams Club, buy their 24 pack of 1# bags of dichlor shock. Each bag will add about 7 ppm of chlorine, and about 6 ppm of stabilizer, per 10K gallons of water. Otherwise, order dichlor from Amazon:
Kem-Tek Dichlor 22 lbs
We do NOT recommend buying dichlor locally, otherwise, at least until you are an EXPERT reader of chemical labels. The chlorinating pool chemicals sold at Walmart, Kmart, Costco, and most other local stores are diluted blends, sometimes with copper and other products with bad side-effects.