Help me please how does ph in soil get so low in wk 3 if i always water with ph 7

AS YOU SEE IN THE PHOTOS YO MIGHT THINK ITS A DIFFERENT PROBLEM THEN LOW PH SO SEND ME YOU INPUT PLEASE ME AND A FRIEND HAVE HAD THIS ISSUE FOR THE LAST 6-8 MONTHS. MY FOLIAGE STARTS SHOWING SIGNS OF WRONG PH ALWAYS IN THE 3RD WEEK OF FLOWER.
I USUALLY DON'T VEG AND IF I DO ITS ONLY FOR 3-4 DAYS i TAKE MY CLONES AND THROUGH THEM RIGHT INTO 2-4 BOXES FOR SOG GROW OR IN 5.5 INCH SQUARE POTS.
DEPENDING ON WHICH WEEK) i ALWAYS USE ADVANCED NUTRIENTS 3 PART FOR MY BASE, WITH ( iGNITOR, BIG BUD, VOODOO, OVERDRIVE, AND FOLIAGE SPRAY WITH SEAWEED OR PURPLE MAXX).
I USE A 8 STAGE DI+UV+PH, REVERSE OSMOSIS SYSTEM WHICH GIVE ME A WATER OUTPUT OF PH 7.5 AND PPM 50.0.
i ALWAYS TEST THE WATER AFTER i MIX MY FERTILIZERS, AND MAKE SURE MY PH IS 6.8 -7.2 WHEN i WATER, WHICH IS HIGHER THAN NORMAL BUT ONLY BECAUSE i KNOW IT WILL DROP. THEY GROW GREAT TILL WEEK 3-4 THEN THEY SHOW PH PROBLEMS. SO THATS WHEN I PULL OUT MY BLUE LAB PH SOIL METER I GOT LAST MONTH AND TEST THE SOIL. IT ALWAYS READS THE SOIL PH @ 5.2-5.6 . SO i USUALLY FLUSH 2-3 TIMES WITH A HIGH PH 8 AND EVEN MAKE THE WATER PH 10.
i USUALLY LEAVE IT FOR A DAY BEFORE TESTING THE SOIL AGAIN AND THE PH STILL READS UNDER 6.
ALSO if YOU THINK MY PH METER IS OFF I HAVE 3 PH METERS THAT i PAID MORE THAT 140 EACH FOR LAST MONTH THINKING MY METERS WERE THE ISSUE AND THEY READ ALL THE SAME WHEN I MEASURE THE SAME SOLUTION., . MY ROOTS AREN'T ROOT BOUND EITHER! AND MY ROOM HEAT HAS GOTTEN HOTTER NOW THAT THE SNOWS GONE BUT I WAS HAVING THE PROBLEM WITH THE COLD WEATHER.
THE HIGHEST TEMPS MY ROOMS EVER BEEN IS 28c /87f. WHICH i ADD AN AC UNIT TO BRING IT TO 80f.

PLEASE HELP ME IF SOMEONE KNOWS SOMETHING I DON'T OR HAVEN'T BEEN TOLD. Ive ONLY BEEN GROWING WORK 18 MONTHS AND THIS HAPPENS TO ALL MY PLANTS UNDER THE 8 LIGHTS I HAVE RUNNING. I EVEN WENT WITH 5 GALLON POTS AND VEGGED FOR A FEW WEEKS BUT THERE SHOWING THE SAME PROBLEMS.
ANY HELP WOULD BE GREAT FULL THANKS

Don't see any photos.

What soil are you using? You shouldn't have to flush with a pH that's that high, either. 10? WIthout pictures it's just about impossible to assess the damage, though

here are the photos I tried posting with my last post but wasn't able. so here they are let me know if you need more to see the problems. OH and I DONT HAVE PESTS I CHECK DAILY AND USE AVID and FLORAMITE IN VEG AND FIRST 2 WEEKS OF FLOWER.

B'cuzz hydroponic mix which i have used since day one

Why the FUCK do you feel the need to post in all caps? Jesus fucking Christ.

This is a most common problem low soil ph or as better know low dirt ph
Quite a problem in sunshine mixes or ffof its has the most posts for problemsIs there a conspiracy going a round to deface the dirt growers or all the dirty companys putting out bad dirt

i been having ph problems going into flowering aswel only my 2nd grow though, found this awesum thread that really cleared up alot of questions regarding flushing soil deficiencys an eveything check it out here ( thread - https://www.rollitup.org/marijuana-plant-problems/437657-

You can solve your problem by adding dolomite lime to your soil to keep your ph stable and supplement with either calmag or epsom salt. That is a magnesium def, PLANTS GOBBLE UP A SHITLOAD OF MAGNESIUM DURING FIRST STAGES OF FLOWERING AND THAT'S WHY YOU KEEP GETTING THE DEF. I would also keep feeding veg food for at least the first 2-3 weeks of flowering.

I just planted 2 new batches this week, and before I even put a plants in the boxes I fill them with new soil watered the soil with ph7.5 and 2 days later just before I added my BC Big Bud, I used my Blue Lab and the Ph was down to 5.3. So what would cause this?????? how much of a raise in temp in the soil cause a drop in ph?? or could it be I have too much or too little CO2 in the room? I have watered with ph 9 and let it run off a lot in one box and the ph stays at 5.2 . I have another room with tables and i was learning Hope to do hydro, (flood and Drain) and if i cant fix this stupid ph issue by my next pull, I pulling out all my 4x6 tables and going flood and drain but I'm having the same issue on them too but my ph reads correctly.
I use AN 3 part and for the first 3 weeks I use equal part of Micro Grow and Bloom, do you think I should boost the grow and lower bloom parts and still add blood igniter??
??

You need to add lime, or some other buffering agent, like oyster shell flour.

Dolomite lime would be your best choice.

pH'ng the solution going into soil does not fundamentally change the soils pH. It will affect it temporarily at best.

Until you add lime to your mix, count on this problem persisting.

Wet

I have been adding Dolomite Lime to the Soil Now. But I dont have the time to find out how much would be the right about to add. can you help? I can either add it to the complete bail which is 3.8cu ft or which would be better I can add it when i fill my box. the size of the box is 2ft x 4ft and I fill up the soil anywhere from 8-12 inch high ( just added 4 cups to these boxes is that enough? or two much?
Also how much should should my ph be when i water with/without nutes.
Thanks