Foliage and root system growing very slow and eventually stopped in hydroponics (NFT)

Water exposed to air absorbs carbon dioxide (CO2). When atmospheric CO2 dissolves into water, it can slightly darken due to the formation of carbonic acid and subsequent changes in pH. This absorption process is a natural part of the ocean's carbon cycle. I'm not saying it's what you have, but I thought it was worth knowing as a possibility.

Hey growmie, just to say up front, I’ve never done NFT myself, but from what you’re saying, it sounds like you're mostly on point. Temps, EC, pH, air stone, all good. But NFT can be tricky with roots if things aren’t flowing just right. If the roots aren’t staying white, my guesses would be: Flow might be too slow or pooling a bit, roots like a thin, fast stream. Some nutrient brands can leave residue, especially ones with root boosters or kelp, that might be building up on the roots. Could also be early biofilm, even if it doesn’t look slimy yet. No pests or real damage is a good sign. I’d clean it out, bump the flow a little, and maybe run a bit of H₂O₂ or add Hydroguard just to be safe.