im testing with a sunleaves ph pen. it says the tap is 7.5.. i only put enough in for what i thought would bring it down by .5 but it went down to 3.4. but then i check the tap water in jug right next to it and its 7.5-8 again. i will say the pen didnt come with calibration fluid so that could be my problem right there. would the pen compensate or anything because of no cal. fluid so it makes my starting reference point be off? im gonna try the vinegar thing and see if that works. thanks