heavy ferting is what lead to the low PH, just ease-up on them for awhile, that flushing you did will help immensly, they look really good, but the damage that is done probably wont go away/get better, the plants will just continue to grow healthy. Check that PH every week, words to live by IMO. 6.6 is probably fine since they were low for so long, you may want to gradually raise it a bit, the 8.2 on top should dilloute over time (as long as they are getting fresh water). use just water with a bit of lime (to raise it a tiny bit) for the next 2-3 waterings and check the PH between waterings.