Hi all! I have a 21,000 gal inground, vinyl lined SWCG pool built last year (now in second season). All is going fairly well, but on my last trip to pool store, water test showed there was some combined chlorine (which they called "chlorine lock" - uh oh . . . ), and water was ever so slightly greenish - they prescribed "shock" a "non-chlorine" product that had an active ingredient of potassium monopersulfate (and was 55% "other ingredients" - yikes! what DID I put in my pool?) That seemed to clear it up for about 10 days, now slightly greenish again. My browsing on this site/forum has raised a red flag about blindly following their instructions.

We began the year with a failure of the salt sensor in the SWCG cell and had the unit replaced, so our chlorine was a bit low at the last test, and our salt is a bit high because the faulty unit kept saying to add more! The most recent water test results (6-1-12) follow:
FC .8
TC 1.1
pH 7.9
Hardness 234 ppm
Alkilinity 86 ppm
CA 40 ppm
Nitrate 26 ppm
Salt 4000 ppm

We fired up the new SWCG cell, lowered pH some and shocked and, as I said above, it was crystal clear for about a week. Current readings by my drop test about 3 ppm TC and 7.6 pH. Any advice anyone has on 1) combined chlorine and the best way to get rid of it; 2) recommendations for a reliable FC and TC test meters - digital - something that will give me a number not make me play "identify this shade of yellow"; or 3) causes of slightly greenish (but definately not cloudy and no sign of algae) water would be greatly appreciated.

Thank you!!