First time coco grow

Day 37: The plants are growing strong 💪 BUT I see some problems, not too much. I still see the magnesium deficiency as I have a very hard time to correct my PH level. I have watered many times with PH 5.5 - 5.8, but the run-off water always show around 7.0~ PH. I am doing some tests atm to try finding the cause. Atm I have 7 cups to test my nutrients/water. I started with 5 cups, measured 1200ml water to PH 5.54, and filled each cup with 200ml of the 5.54 PH water. The first cup I did nothing, 2. has 1ml of Canna coco a and b added, 3. cup 1ml of Rhizotonic added, 4. cup 1ml of Cannazym, and last 1ml of BioBizz CalMag. I then tested each cup PH, re-tested 10min, 30min, 2hours, 15 hours, and 24 hours later (So far) and all cups have increased PH level. The CalMag cup was the worse with an increase of 1.7 PH (went from 6.16 to 7.87). Even the first cup, only containing tap water (PH'ed with acetic acid) has increaed PH of 1.05 (went from 5.54 - 6.60). This seems very odd. Is there a problem with the tap-water? I decided to test my water against some store bought. I have only just started the test and I don't have the results yet. We must see tomorrow Day 38: Nothing makes sense to me anymore regarding my PH tests.... Every time I test a cup, no matter with nutes or not, with tap-water or store-bought, the PH increases on every test, even if I test them after seconds. I was expecting small +/- in my readings, as when I have tested 1 cup I test the next, and some small drops might land in the other cup, although I try my best to clean the meter between every test. I am clueless here.... Plain tap-water has increased ph from 7.85 to 9.10 in 44 hours. Store-bought water has increased PH from 7.84 to 8.70 in 20 hours. Can anyone explain me what is going on? Any idea what I could do? I did test my meter by making a new mix of water and acetic acid and got PH 5.5~ and than tested the old tap-water cup to the same as on my schema, and than the new mix again which showed about the same 5.5~. The meter might not be as accurate as the manual tells, I would say its maybe 0.2 +/- but defiantly NOT +/- 1.0+ Day 41: Plant have entered flowering state and I still have problems with the PH and now also EC. My tap water is showing 1.0 EC now. I have ordered a couple of things to make sure my PH and TDS meter is working properly and have to wait for it to arrive. Until then, I won't do anything, unless I have to. Hopefully the stuff will arrive before they need water. The plants are still growing fine, but I see some problems. I still have small signs of magnesium deficiency. I also see a little nute burn, properly caused by my suddenly high EC in tap-water (but not sure yet). I am not sure if there is other problems too, there might be, and I think it will be more easy to diagnose when I know the correct PH