We've had one of those round Intex pools for 5 years (10' for 3 yrs, got an 18' last year), and never did anything but use chlorine tabs in floaters and occasional testing and shocking every 2-3 weeks as a precaution -- never had any real problems. I think the first year we got some brown color after initial shocking, but the filter took it out in a day or so (I might have used some softened water, that first fill).
The past few weeks, we had a some very hard rains -- the last was 2" in less than an hour! -- and some hot days (90+). A few days ago, the water became a bit hazy, so I shocked it (1 lb). The water got so cloudy you couldn't see bottom, and the pH went from 7.8 to 6.4, so I added a 5 lb container of "pH Plus" (sodium carbonate) -- whereupon the water turned a solid, glaring, milky white! pH when to 7.5.
I've been filtering constantly for two days, and get white residue when I rinse the changed filters, every 4-5 hours. The water is a bit less white, but not by much. Definitely cannot see bottom, or even the sides! It otherwise seems clean, there is no smell, it feels OK.
Last year's test results were:
pH 7.5 - 7.8
pH was 6.8 - 7.2 this morning. Also, FWIW, we have a shallow well, which is hard enough that we use a softener for house water. The pool was filled with raw well water (over a 2-week period), as in the past.
I don't know what to do from here...