Hey man 
I added bleach the last time I did dwc, seemed to do the trick. Problem is you have to add it every day because it evaporates out 

I bought a chlorine tester that measures in ppm so you can keep track of what's in the system. I think anything over .1ppm is playing with fire though (from what I've read)
It only took +-2ml added to 15 (US) gallons of ro water to bring me to .07 ppm
As far as the plant being too big for its roots, I think that's impossible in water bro. Hahaha
If it needs more roots it'll push them out. I grew a couple trees in dwc totes and the rootball could fit in a shopping bag
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