Ph Issues In DWC

OK.......new to DWC........done a few ebb & flow grows.
Started 10 sprouted seeds in 5 gallon DWC buckets. Only one rooted and took off. A lovely little California Orange Bud seed from UK. Not feminized.
It began life in tap water with AN Micro/Grow/Bloom nutes, with organic additives. It took off quick!!! Under 600w Sunmaster MH 18/6. Plenty of aeration. Starting pH was 5.8/TDS: 300.
Room stays between 66 and 76.....humidity between 35 and 55.......bucket temp between 18 and 22 celsius..........
Unfortunately my pH/TDS pen crapped out and I had to test the pH with a dropper kit......it was somewhere in the 5.0 to 6.0 range for the first week.
After the first week of good veg growth, the organics started making my nute bucket frothy, and being a pythium survivor, I wasn't gonna risk it again......
I did a Clearex flush and eliminated the organics.....adding in H202 @ 1.7ml/l with the M/G/B. pH was between 5 and 6 when new bucket set up.
The next day the plant looked wilted. Just not perky like she had. Roots still looking AMAZING. I received my new pH meter that day and tested water........4.3 pH...YIKES!!!!!
I decided to go buy some RO water at this point and do a bucket change.
On 1-28, just before lights out, I changed out the water with RO (pH 6.55/TDS 20) and added in fresh nutes, plus a cal-mag supplement. 2 drops of pH down brought pH to 5.73..TDS 690..water temp 18.5c.
1-29----plant still droopy.....otherwise looks green and starting to smell a little skunky. 6" tall and 7 nodes. Little bush.........at 1:00pm I tested pH 5.32/temp 22.1c.....added 1 drop ph up.....at 2:30pm pH 5.63.....at 10:30pm ph 5.4/TDS 648/temp 23.4c........
1-30------today she is still droopy and the largest fans are yellowing slightly. at 6:30am the tests showed pH 5.6/TDS 548/temp 20.6c.........at 1:00 things got weird to me.....and that is why I am writing......when I checked her the yellowing on the leaves was more evident, and the tests showed pH 5.36/TDS 754/temp 22.2c......WTF???
How can the pH drop that much and the TDS jump that much in only 7 hours???
And why the yellowing???
And the wilty look??
Should I add pH upto get back tooptimal ranges or leave it??
Does DWC lend itself to such pH shifts even with RO??
Someone some help please????

Last night @9:00 I checked and the pH was 5.3. I added 1/2 quart RO water (pH 6.5) and waited 15 mins. tested pH and it was 3.5. HOW THE FUCK DID RO WATER DROP THE pH??? Meter is brand new and I recalibrated it after the test, and yes the pH was 3.5.
I added 1 1/2mL Barricade and got pH up to 5.2. Then added 1/4mL pH Up and got pH to 6.2. Saw an immediate perking up of the plant. 2 hours later the pH was 5.9. This AM pH was 5.8.
WHY IS IT DROPPING SO???? Temps in bucket swing between 18c and 24c.HELP!!!!! PLEASE!!!!

Wow man im sorry ur having so many problems.. I use tap water currently as well as organic nutes..mix my solutions and ph down to 5.7-5.8..Calibrate my pen once a day and check..Day after res changes i may get a lil swing and of course during top offs but mine stays relatively stable and havent had these problems u speak of..
U must mix RO water properly if ur going to use it. Earl is the ro guy.
If u had a pic of the plants it would be easier to determine what the problem is.

I mixed the RO up the way Roy at AN told me to........adding the nutes in certain order, stir after each nute added, allow to settle, etc.........the plant has an obvious N deficiency (no surprise there with the pH levels) and since getting the pH above 5.5, there is NOTICEABLE improvement, but the pH is dropping again.
I don't like to post pics....paranoia....and besides, these are all lies anyway, so I don't have a pic!! MMMUUUUUU WAAAAA HAAA HAA HAAAAAA!!!!
BUT.....I would send pics if I thought I might get some real help.
BTW...the roots looks great......whitish beige and lots of them.