Converted saltwater to fresh, trouble keeping PH down...

Hi All! So I decided to change over my saltwater tank to a freshwater tank and now I cannot keep my PH down(7.0). I have a 65 gal. Red Sea Max 250 so all the pumps, filters etc are all built into the tank. Inside the tank and filter area we sprayed vinegar on the mineral deposits and salt to loosen and remove it. The sponges/filter media, a pump and any loose parts were soaked in vinegar, scrubbed then rinsed to remove deposits. We removed the protein skimmer all together and replaced one of the two pumps. Emptied all the water and removed all the sand except maybe trace amounts and refilled it about 7 times letting it sit in between do dissolve any salt etc.

Then we filled it, waited a few days got the temperature up and the PH was about 8.0. so we did another couple water changes.
Last night after filling my tank I tested it right away, it was still 6.8. I added "Neutral PH 7.0" just in case. (Note: Water straight from my tap is testing about 6.8.) Then, 24 hours later it has jumped to at least 7.6 (that's as high as my chart goes It may actually be higher.) Does anyone know why the PH would jump so high, so fast? Update: Added "PH Down" it lowered from 7.6 (or higher) to 6.8, and overnight its back to 7.6 (or higher).
Is there something left over from my saltwater tank that would cause the pH to increase? Would leftover mineral deposits that we missed cause it to increase? There was only trace amounts of sand left so I doubt that would be it.
Any idea how to get it down and stay down. Thanks in advance.