500L res! close to 130 gallons....Doesn't matter - It will NOT hold pH for even one day, let alone a week. It can't be done!
Not even close....the average daily adjustment would be like 100ml to 160ml of down give or take.
Your res change of 2.0 and in rockwool is indicating that the rockwool was not conditioned enough/incorrectly and is effecting the pH...
Did you condition the rockwool? How?
You may "feel" your giving all the answers here. But with out some answers to our questions, we cannot give you the quality answer you want...
Let me say this again. You can use all the science of water chemistry you want. You WILL have to monitor and adjust your pH DAILY! THERE IS NO WAY AROUND IT! Google all you want, it will not change this!
If you go to RO. You WILL have to add a Ca/Mg at approx 5ml a gallon to that! Just to make the plant grow without defs. (now you've added a buffer back)
Sorry, but if you think that growing can be a "lackadaisical" affair. You will NEVER master growing Cannabis! You cannot bend hydro to your will.......Bottom line.....
Your Rockwool was not conditioned properly. You should be seeing a pH swing of like 0.2 - 0.5, Maybe 1.0...
Your getting a pH swing of 2.0 and it swings back too fast to be anything but a problem with the Rockwool...
All Rockwool is not created equal.....If it's not Grodan and it's a cheap alternative......THAT could be the problem too....I've seen this myself!
Doc
Dude im actually quite enjoying the discussion now and on those very rare ocassions its a shame we cant be face to face blazing a doob maybe chat about some hot chicks and get to the bottom of this. My point being the internet sucks!
Theres so many points here that its hard for us both to get a word in ( that and more than one thing at a time and im fucked
) so lets go with yours first and thats 'Cube Prep'.
I buy and order grodan on line and from my local hydro store, they are registered sellers and all 1.5 cubes come in white 24 cell holder trays with a plastic clear sealed outer sleeve and paper insert showing a bottle of clonex and a cube with what looks like a clone of some broad leaved salad leaf or somthing idk. The back of the paper insert has pics of formulex clonex and some zyme or somthing, i always throw them and dont have one around. It also has prep and how to clone advice. To prep they advise a soak in formelex for short time ph 5ish etc.
Nothing i have done i.e. Ph, soak time, multiple flushes and soaks, ec or just ph tap has ever made a difference and ive been through 50 in last few weeks easy, a simple soak one hour gave exactly same results as the rest.
What did make a the only difference was getting the watering right. Below is a plant i half got right and a seedling in front that got overwatered (even though i thought id got watering right!). I know the seedling was overwatered because it germed in 3 days from seed only to halt growth when first true leaves were tiny, its growing better now but i caught it turning away from the light for couple days then bingo i watered less kept cube less soaked and it started reaching for the lights again.
I always seem to have over under water issues and hard to get totally right till a larger volume of roots grow. Those white cell holders keep the side and bottom roots moist when water is low nicely. I use a unheated propogator to till seeds pop and dosent seem to matter how wet the cubes are in the propogator.
Another thing ive tried lots of is no ferts vs ferts, every bit of litreature, bottle of ferts, cube packaging recomends average 0.8ec pre soak germination ans small seedlings, mine also seem best here and believe me they take off as oppose to plain watered ones theres just no dispute for me.
I cant detail things more and dont think think this stage is a problem for me plus ive run most ways possible and just gone with what gives me results. If i get watering right there is nothing but green but when i mess ul its purple stems and brown roots. Pics speak more than words and your average noob aint growing seedlings in rockwool as nice as mine generally but then again im still screwing up worse than a newb witb that stunted pale purple seedling in front, rootz are so fragile in this small cube at a young stage its easy to fuck up big. I hope that sort of ok plant in the background is showing you im close and shows how the main problem in rockwool is mainly watering.
Cubes are not in a res, ph is maintained daily so i dont think it will become an issue till i havea res going.
Btw i own most good ferts, ph ec meters lights and temps humidity are all great all the time.
There is NO solution to your request! 
