Hey TerpKing, has the PH stability improved at all?
Hard to imagine that the pebbles could be to blame after all this time, isn't it?
I also wonder how empty your flood tray gets when it drains. Is it possible that a significant amount of nutrients stay in the flood tray?
(Is the tray nice and level or pitched toward the drain?)
Then, maybe after you adjust the reservoir to where you want it and then it mixes with whatever nutrients were sitting in the flood tray at the next flood - that changes the PH in the res???
I have recently had an interesting PH experience. I started using an RO a couple weeks ago. Previously, I had been using well water and using PH down to keep PH correct.
After switching to RO water, I now only ever use PH up to control the system. Wasn't expecting that and not sure of why... but my stability is decent. I make adjustments every 24 - 48 hrs. Still getting used to RO but not struggling to control it too much.
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Hey hey EBnFLO.
I'd say it's slightly better now, not how I'd like it though or how i think it should be. I have to re-adjust the PH on a daily basis before first flood. If I PH around 5.4/5.5 it will be up at around 6.2/6.3 when I go to re-adjust it again before first flood the next evening.
I'm emptying the reservoir fully this evening and switching over my nutrients as this evening will be the start of week 4 flowering.
I'm going to do a little test. I'll take a few of the clay pebbles out of the tray put them in a glass and add some of the nutrient mix I've taken out and record the results of the PH. I did do this before and the pebbles were definitely making the water in the glass PH rise over time. I didn't run the system before hand, which I should have done before putting the plants in. I just cleaned everything really well and thought that would be enough.
I'm thinking that my clay pebbles are just sitting at a higher PH than they should be and as I'm using R/O it does not take much for the water to fluctuate and change PH. I'm not really 100% though as this is all still new too me and I'm still learning.
When the flood tray drains im assuming most of it emptys as it is slightly pitched down towards the pump and the overflow pipe, once the timer turns off you can hear the last of the mix running back down through the pump into the reservoir. There could maybe be a little bit sitting up there tho, I'm not sure I'll dig my fingers around in there later when lights are on and find out.
I've noticed the biggest bump in PH is after the first flood where it can go up by .3/.4 then it will slow down going up only maybe .2 a flood. It's very strange and annoying. I'm thinking it shows down as it's getting closer to the PH of the pebbles, again I'm not 100%. I'm sure when I was using normal water from the tap though the fluctuations were much smaller and took far longer for the PH to rise, this was using different pebbles tho. So not sure if my problem is with the R/O or with the new pebbles as this problem started when using both which were new. Some people mix some normal tap water back with there R/O water, I don't know if that would help as it would add some more buffers to the water.
Very interesting indeed, I also used to have to lower my PH when mixing up my nutrients when using tap water, but since switching to R/O I have to bring the PH up after mixing my nutrients as it's to low otherwise. I only have to re-adjust it down after the mix has been circulating through the system. I'm adjusting about every 24 hours too, 48 would be nicer though. Glad you're not having too many problems pal, I've managed so far to keep it undercontrol, it's just taking a tad more effort than it should do to do so. Be interesting to see what happens with the glass pebbles test later!
. Definitely going to give coco ago. Either in smart pots or some of system to drip feed. I'm thinking hand watering once a day could also be pretty easy. 
