PH is being stubborn

Ok so this winter i had such a good time growing indoors i decided to let the sun boost my yield. I built a NFT kinda system in a tube (pretty much whats used for septic pipes or maybe really big electrical wires) with 7 - 5" net pots. The whole thing is filled with inert pebbles.

The pebbles have been rinsed and dried and rinsed a few more times, the tube was blasted with a hose, and the hose used to bring the nutrient/water up and what gravity feeds back down is all new and clean.

Im just starting out with my plants being maybe 6 inch tall i have 3 tomato and 4 bell pepper so the ppm's are only around 600 but I cant seem to get the PH under 7.4!!!

At first not knowing how much i needed to keep in my tote so my pump didnt run dry i filled it with a hose till the pump turned off, marked it, and used a empty gallon jug to measure how much it would take. Me not thinking about only using weak nutes filled it back up with tap and PH was way high. So i empty it all out and go buy 8 gallons of distilled water which i added the nutes to, tested it, and did the math to figure out my nute needs.

Well now im still at 7.4+ (even tho i tested it at 5.8 before i made all the jugs). So what do i do, i go and buy some PH down from the pet store made of sulfuric acid, i pull ouy a gallon and 2 drops works like a charm instant success!!! So i do the math, add in 14 more drops and test.....its 7.4+ again. Ok ill wait a few hours and double it up...still 7.4 So now im spraying this stuff like hot sauce into a giant chili and it starts to change wahoo its yellow, 2 hours later its blue again what in the world am i looking at here???

Sorry for the short book i just figured id let all the details be known so to lessen the questioning tag =)

Bump ..........

Sounds like the pebbles may be buffering the ph back up.
Make up a small batch of nutes and put half in a container with pebbles and the other half without pebbles.
Let it sit overnight, compare the ph and see if theres any difference between the two.

Pebbles are inert =(. I finally got it to somewhat stablize. At first i thought it was the leftover water attached to the pebbles inside the system (it runs 10 feet so thats alot of surface space hehe) But when i drained out the rez it was almost at the lvl it was when i filled it, telling me that 8gal of proper PH should handle it lol. But with every problem heading out the door one will slip in, now i have spider mites lol. Well atleast the things outside so maybe bugs will help, but considering theres atleast 10 in plain sight i might have a bigger problem then they can handle. I dont have any kneem oil and i refuse to pay 10$ shipping every dang time i order online so im going to stick with rubbing alcohol diluted 3:1 and hope that works if not i guess it isnt too late to get some new plants. Any other ideas would be great ima go hose them down with cold water Thanks in advance!!

Ok Ive managed to keep the PH stablized long enough for these suckers to ask for more juice!! Going from 300ppm to alittle over 1K now the PH is always on point, cant wait till this seasaw starts going back down and i need PH up hehe. But the tomatoes have gone from being completely stunted and ticked off to about 3 feet tall in about 3 weeks, they really love that sun. Now my newest issue is keeping the temps in my rez cold, i throw about 2 gallons worth of ice in it a day. I have the rez burried about a foot + to try and use natures way but seems its not enough when it hits 90 degrees. Might be time to get the shovel out and handle this like men. Anyways ill try and get some pics up tonight so you guys can get an idea, but i think im gonna end up having to buy some kind of cooler. TTYL

Oh for future reference i used sulfuric acid (fishtank PH down) to stabalize my PH, works like a charm just make sure that yougo back every few hours and add more till you find it where you want it. Weird but it seems like Atomizer was right , i feel as if these pebbles acted as a stabalizer to keep from PH bouncing/ that or the nutes not sure. But once i got it stable id only have to add alittle bit each day to keep it there, and i mean like a half a top full for 9 gals. Hope this helps