First off...don't believe a 15 sec test strip--it may be accurate, maybe not. You can get a little OTO/pH test kit at any pool store and most discount houses, even that is better.

HOWEVER, the one thing they are usually pretty good at is chlorine--if it shows 0 you have 0. If it shows 10+, you have 10 or more--between those two....weellllllll I don't buy.

I know I'm being a little glib but if your pH is 6.2 you need to raise it immediately. Your two best choices are Borax or Washing Soda/Soda Ash. Borax will raise pH, and Soda Ash will raise pH and Total alk, which you need (but you can add baking soda instead).

I'd start with a full box of Borax --20 Mule Team at your grocery store. Make sure the pump is running. Add it to the skimmer. Wait, say, an hour. Then I would add, to the skimmer, 1 gallon of regular bleach to get some chlorine in the water. Wait, oh, say 2 hours and check your water again. If the pH is below 7.0, add another box of Borax. Wait another hour and if FC shows less than 3ppm, add another gallon of bleach.

Keep doing this until pH is 7.3-7.8 and FC is 3ppm.

Then worry about total alkalinity and CYA (stabilizer). You are aiming for T/A of 80-125--add one # of baking soda, wait a couple of hours and check. If low, add another #. You need at least 30ppm of stabilizer, but not more than 50ppm. You can buy this at discount houses or pool stores. Figure out, from the label, how much for 13,000 gallons (24'round) you need to get 30ppm of CYA. Use about half that amount. You can put it in an old stocking and hang it near the return, or put it in the skimmer, but don't turn off the filter or backwash for a week. Then measure CYA again, and repeat, until you reach 30-50ppm.

Hope this all helps.