I'm still thinking the problem is that you're using soil nutes with soilless. You may be overfeeding, or they're just inappropriate for an organically inactive medium, contributing to the peat acidifying (which it's prone to do).
It's confusing like using hydro nutes in soil. Do you target 5.9 or 6.5? (is it soil or soilless?).
Since the runoff ph is so low, I would target 6.5 to help hold it higher.
Also, the visual could be partly nute burn, which could contribute to the "buildup-like" acidification of the runoff. The nature of the nutes might compound the mismatch between soil nutes with an organically inactive soil.
I would reduce the nutes. That would rule out nute burn and might help with the other possibilties. (And target 6.5. Even higher if it continues. When I had a acidic soil problem, I p up'ed to 7 sometimes.).
It's confusing like using hydro nutes in soil. Do you target 5.9 or 6.5? (is it soil or soilless?).
Since the runoff ph is so low, I would target 6.5 to help hold it higher.
Also, the visual could be partly nute burn, which could contribute to the "buildup-like" acidification of the runoff. The nature of the nutes might compound the mismatch between soil nutes with an organically inactive soil.
I would reduce the nutes. That would rule out nute burn and might help with the other possibilties. (And target 6.5. Even higher if it continues. When I had a acidic soil problem, I p up'ed to 7 sometimes.).