Ph in res keeps rising please help

Ok so I use a res simply for the convenience of having stored water/nutes for my ladies. I do all organic soil grows.I'm hand watering with a wand connected to a hose and pump in my res. I haven't added any nutes yet because I'm only in the second week of veg. I used to ph 5 gal of water/nutes at a time without a problem. Anyways I'm using the res this round filling with 30 gal of phd water at 6.3 when I go and check the ph the next day it always raises up to 7.0 or higher. I use earth juice natural up n down and I wait about 30 min b 4 taking my first ph reading using a good quality ph meter. the water is being circulated with a sub. pump. How come 24 hours later it rises so much? Any help will b appreciated the unstable rise of ph scares me cuz I don't really know whAt the ph I'm feeding my ladies is it's fine when I feed em then the next day the water in the res is all out of whack . Thanks

Also I'm using tap water double filtered with a hydrologic small boy by the way should remove 90% sediment and 99% chlorine I used it last round with the 5 gal jugs with no ph problems

I am new to hydro as well and the main thing that was affecting my ph rising was the temperature of the rez. It used to run around 72-75F and another member told me to use ice packs.
I dropped 1 ice pack in at lights on and took the old one out. I can't tell you how much it has helped! Do a search for rez temps but mine stays around 65 and when I check ph, its always within a few 10th's from when I fed. Good luck man!

Well I don't do hydro I'm all about the organics n soil but I use the Rez because it makes it e z to hand water and i can store bigger batches of my nutes but that's a good idea the temps might have somethin to do wit it I'm on 24 0 right now for veg so there's not much fluctuation in temps of the room but I have been ph ing while the water is probably still cold I'll try leaving it there a few hours b 4 I adjust ph so it can warm up to room temp mayb that is the problem?

N e 1 else think that's the problem?

Well I don't do hydro I'm all about the organics n soil but I use the Rez because it makes it e z to hand water and i can store bigger batches of my nutes but that's a good idea the temps might have somethin to do wit it I'm on 24 0 right now for veg so there's not much fluctuation in temps of the room but I have been ph ing while the water is probably still cold I'll try leaving it there a few hours b 4 I adjust ph so it can warm up to room temp mayb that is the problem?

N e 1 else think that's the problem?

I did the same thing, using a 18 gal tote to store water.... without a good buffer in the water, your PH is going to fluctuate. Once you start adding nutrients, they should assist with stabilizing the PH.

Thanks guys