New Reservoir - pH up - PPM down - why?

Has anyone else seen this?

I make a fresh batch (in extra container, no airstones or anything) of:

6 gal RO water
18-ish teaspoons Flora Nova Bloom
3-ish teaspoons Sensi-Cal (cal, mag, micros)
1/2 teaspoon 30% H2O2 (a daily additive)
3 teaspoons SM-90

Right off the bat, the pH is 5.0 or lower, and PPM is 1600+. I know from experience that after adding it to the live tank (with airstones and plants), after several hours the pH bumps up AT LEAST .5.

This time, the new batch started at pH 4.8 and 1650 PPM. So I bumped it up with some ph up to 5.3. I put it in the live tank about noon yesterday. By this morning, the pH was up to 6.2 and PPM was down to 1200. WOW. I just hit it this morning with a little pH down. Later I will do my usual H2O2, then whenever the water level goes down a bit more in the next couple of days, I will add some more nutes to bring the PPM up closer to 1500 (which they seem to like).

What gives??? Is the H2O2 adding a lot of PPMs that then go away? Is it also temporarily lowering the pH? I know I am not supposed to expose plants to >0.5 pH swing at a time, but if I don't, I end up at 6.2 or 6.3 the next day, and have to bump it back down again.

How old are the plants and how good are they growing?

Sounds like the plants are just eating nitrogen.

Sorry for the late reply - had to find the journal. They are 7 weeks into 12/12, and I *think* they supposed to be *long* flowering - definitely sativa dominant. Only one is good (pictured), with 1 giant main and 2 "sub mains" that are just outstanding so far. Out of the other 2 plants, one is way dwarfed and one was WAY overgrown - I tried LST'ing the crap out of it but it was determined to put everything IN the lights. There is no significant bud on either of the other 2.

They all took a while to even show sex (3 weeks of 12/12).

If your PPM are going down assuming you aren't adding lots of water - your plants are eating those nutes so you need to add them back in.