Hola & Feliz Navidad y’all!
This Purple Glue seems to have some problem.
I guess it’s maybe too much Iron, but i tought it’s always clever to ask the swarm.
In reality it looks a bit more brownish then on the pictures.
Thanks in advance 
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It looks a lot like nutrient burn/salt buildup in soil.
Try checking the PH of the soil near the root mass and see if it is above accepted levels. If so flush your plants and use less nitrogen in your feed.
This is only one of the many scenario’s of what could be causing your problem but it is where I would start to look for the problem.
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Thanks for the quick reply @Oldjoints !
I feed from the bottom and the pH is always 6.5.
Maybe you’re right as i use one reservoir for 12 different plants and the rest looks fine.
Well then she have to suffer a few weeks 
Thanks 
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