Has anyone here ever used coco coir drip feed auto...

Yes u can I don't recommend it might be to much trouble to be worth it. You will have to measure ppm and replace what that have eaten. But how will u know what npk to replace? Cuz u won't know it the plant took in more nitrogen then phosphorus and what's being returned for example. If u do just make sure u monitor everything closely especially what your plant is telling you. Good luck 👍

The reason it's not reused is there's no way for you to know which or how many macro and micro nutrients are being held by the coco.It has to deal with the positive and negative charge of the coco and the +,- charge of the nutrients. Reusing the nute solution could lead to deficuencies showing up not to mention unwanted bacterial growth of a harmfull strain could be growing in your nutrient solution. Hope this helps.

Never tried it ! But I have heard that coco is a medium that shouldn't be full of water , you usually water then wait the medium to dry a bit to water again. Over watering is rare in coco but can sometimes be an issue so be careful about this method. I think that the issues are not nutrients related but more about the roots not being in a medium constantly full of water.

I run a recirculating system but my medium is perlite in dutch buckets. I dont see why you wouldnt be able to recirculate with coco but You will have to keep an eye on your ph and make sure to keep a lot of oxygen pumping into your reservoir. I am not sure it would be very beneficial in coco since it retains a lot of moisture but hey nothing wrong with experimenting with it.