It doesn't seem likely that you had that dramatic of a change in ph in one day. My guess is that there was either a testing error on the 7.8 reading or on the below 6.8 reading. (You haven't added any muriatic acid have you?) Retest your ph again and see what you get. If it is indeed below 6.8, it may take quite a few additions of Borax to bring it up. When I add Borax, I usually wait about 3 or 4 hours before retesting and adding more. When you see the ph begin to rise, you should begin adding smaller doses than a box at a time so you don't overshoot in the other direction. As far as whether you should aerate the ph up or use Borax, in this case, if the ph is indeed below 6.8, you should use Borax because you want to get it above 7.0 ASAP. Readings below 7.0 are acidic and can damage your pool.

Also, you should be running your pump 24/7 while trying to clear your water. The cloudiness is most likely dead algae that may take some time to filter out. Watch your filter pressure and when it rises 8-10 psi over clean filter pressure, backwash.