It ended up that only one plant was affected by going limp. It was the last plant near the overflow in the more mature tube, and has since been removed. Roots looked the same as all the others, nothing else seemed unusual. Even the plant right next to it was fine.
I finally did a full
nutrient replacement (20 gallons) and cleaned out the reservoir and
pump. Flow rate is higher now as a result of the pump cleaning.
Yes, there is a mixture of young and mature plants in the same system. So far they seem okay, but it makes me wonder how to properly handle rotation. If I start some new ones now to help insure a continual harvest, it sounds like I should have two systems: one with 1/2 to 3/4 strength
nutrients, and one with full.
All of the plants look amazingly healthy right now. No colored edges on the leaves, full heads, etc. on all 25 remaining. But the roots are looking a bit more brown than I would have expected. This seemed to start about ten days ago. Everything else seems fine: strong leaves, no signs of mold or algae, reservoir is clear (smells "organic"), temp of the solution remains cool to the touch (never close to warm). I started adding a bit of hydrogen peroxide (3% with H2O2 and purified water) two days ago even though I have an air pump rated for 20-60 gallons. Any other thoughts, or am I over-fretting?
Roots are a little discolored, not sure why yet
Everything else looks great, and we're starting to pull leaves off to eat!
Oak Leaf, 36 days
Buttercrunch, 36 days
Lots of green!