PH and PPM drop BEFORE using nutes?

[Klaatu]

I'm using FFOF soil in 3- 5 gal. buckets
strain is Pure Power Plant sativa dom.
R.O. water, GH flora Trio (yes I know they're hydro nutes but they CAN be used in soil) but after this grow, I'm switching to Fox Farm nutes. I also use Cal/Mag 1 tsp. per gal. and Hydroguard, as well as a little Hydrogen peroxide.
4' X 4' X 7' tent
600 W

I made up my nutes with ro and PHed it to 6.5, but I waited till the next day to feed them. I thought it might be a good idea to check the PH and PPM again before using the nutes. I was suprised to see the PH at 5.5 and the PPM that was at 900 is now 735?
What's the story, should I just raise the PH and use the nutes?

[Californiakid]

I haven't used h2o2 in my mix yet but I would think that while sitting over night the peroxide gave off some oxygen in turn concentrating the hydrogen lowering your ph.

Now I don't have the experience to suggest a method of action. Hopefully someone else can finish up the loose ends.

Good luck