Thanks for the feedback guys, but.....aaarrrggghhh! My brain is mush. The comments from both of you got me back to reading about buffers again, but as usual, I got lost down the rabbit hole.
One site that keeps popping up in my Google searches is this one.
Chemical Buffers in Hydroponics : What is the Best, Cheapest Buffer
This is the guy who wrote the HydroBuddy software. I have it installed, but have not figured out how to use it yet.
He has a few ways to handle PH swings, but as far as buffers, he suggests that using MES is ideal, but $$$. The next best is carbonates, but I cant figure out what exact carbonates he is talking about.
It sounds like phosphates and potassium are both potentially toxic if too much is used, and citrates just didnt work that well.
From the link above:
The red and yellow curves represent two solutions with increasing levels of carbonate showing us that if you are battling pH increases, having more carbonate will definitely help you deal with this. However it is also clear that carbonate concentrations at pH 5.8 are restricted to around 100 ppm since values above this are bound to cause toxicity due to the very large presence of the hydrogen carbonate ion. The green curve represents an increase in the amount of phosphate from 0.002 to 0.004M (about 100 ppm) with carbonate, showing us that phosphates are not good at buffering increases towards the upper side but they do increase buffering towards acid territory. Overall I also noticed that citrate concentration increases to the maximum threshold allowed by calcium citrate solubility did not afford a very good buffering effect with only a mild effect that prevented shifts towards the downside.
In the end, the conclusion seems to be that in a regular hydroponics system where pH increases generally happen towards the upside it is better to use carbonate as a buffering agent than to use citrate or phosphate although phosphate at its regular concentration in hydroponic does provide some buffering against pH moves (without phosphate increases are much more dramatic). For this reason I believe that a phosphate/carbonate buffer seems to be the best choice for most hydroponic growers, taking care to keep the concentrations at levels that do not cause precipitation or phytotoxicity problems.
BUT - then I read another of his blogs about using ion exchange resins.
How to Have a Constant pH in Hydroponics – No More Corrections ! :o)
This sounded perfect to me. The resins are very $$ to buy, but they can be re-used up to 100k times and are easy to re-fresh - just soak in vinegar and wash. So that brings the daily cost down to a very reasonable level....maybe.
However, finding some in a small quantities is not easy. He says about 1 gm/gallon of water is enough, so I only need 20 gms or less. Ive managed to find some sources, but they either wont sell to me or they are in Europe with 6 month lead times or they are out of stock or the minimum quantity is 200 gms for $500, etc.
The other issue is everything I have found so far is very fine powder - 70-150 uM, so I will need a very fine mesh bag or container if I want to be able to re-use it and not loose it into the system.
So, ion exchange resins might work great, but you cant get them and the particle size is too small to re-cover them easily....
Im still looking though.
He has some other blog posts about mixing nutes so they are somewhat PH stable, but I havent gotten to those yet.
On a related but slightly different topic - the Advanced nutes just cant hold PH for more than 2 days before it starts to go up fast.
The UV light seems to be at least as effective as the chlorine as far as preventing scum and precipitation, but the PH still shoots up after roughly two days and I have to start PHing it back down.
However, I noticed a few days ago, that when I forgot and left my pump running full time, that the PH went up much faster and sooner. I forgot about that until today. So I have changed the re-circulating pump from 1 minute ON and 5 minutes OFF to 1on/30off. So far, it looks like the rate of increase is less when I circulate less, but its too soon to tell for sure.
That seems to say that I also have a carbonate issue - even with the RO water, and that circulating the water is causing the PH to go up due to C02 exchange - I think. Maybe.
Im going to watch this for a few days to see if the PH swing gets better with less circulation.
On the other hand - the plant and the roots still dont seem to give a crap that I am having PH issues. Other than slight signs of tip burn (EC too hi), and some purple on some leaf stems (Mg def?) they look great and are growing just fine.
However, I CARE ABOUT THE PH!!! Stupid plants just dont understand that PH needs to be IN THE CORRECT RANGE or the world will end!!!!!!!!!!!