Hi Pedro, welcome to the forum!! I think you should first rule out the sand as the ph culprit. See if you can get some distilled water at the store which should have a ph of 7.0 (check first and make a note of the ph when you bought it.) perhaps buy a gallon of distilled water (roughly $1) and pour half out, then add a couple handfuls of sand and put the top back on. let it sit a day and check the ph, if its higher then your sand is the culprit.
if the sand is not the culprit and your water is settling at 8.8 after aging, then an RO unit may be your best bet.