Co2 is naturally acidic,and aerating the water would drive the Co2 off raising your Ph levels.
Out of interest what are you using as substrate in the various aquariums.
Water companies also use temporary buffers to alter Ph, which when this has been standing for 24 hours you will get a totally different reading.
Just take a small sample of your tap water, test the Ph,and then again 24 hours later, 8)
