I have used Foliage Pro, and still do for some things, in my NFT before and had no issues. Your pH when using RO shouldn't be constantly going back since it's not buffered by itself and any buffering from the nutes should maintain fairly well. Have you had the same issue when using any other nutes or just Foliage Pro?

Well I've only tried Foliage Pro, which is why I asked if any of the other formulas were more suited to hydroponics. I had a thought which is that I'm using the FP at a reduced concentration in an effort to avoid nutrient burn on my seedlings, and that perhaps this is causing unstable pH. Any thoughts?

What type media are you using? And what type system?

If you're using a very low concentration it's possible the buffering from the nutes can't achieve stability of the RO water but once you start adding pH down to it it should become more stable and hold just fine. Well, barring any other issues.

I'm using Hydroton in a DWC setup. It could be that the Hydroton has not properly been presoaked in pH 5.8 water. When I first set my system up, I just rinsed the Hydroton and my pH would shoot up every day. Then, I stored it in water around pH of 6 for a while, but maybe I should have done that at 5.8 instead. Think this could be the culprit?

Looks like it may, in part, be due to a super low concentration of FP. I was so worried about burning my seedlings that I didn't use hardly any. I added more to my reservoir today, and as I was adding it, I watched the pH go from 6.1 to a hair below 5.9, which is right where it should be. I should've known better than to doubt Dyna Gro!

How much of fp do you add? I was wanting to try 'passive hydroponics' with peace lily & mabey bromolaide to see how it does. Already got the clay ball things but I kept hearing about 'nutrients' to use. Was hoping I could just use my foliage pro I already have.