The cutout you made in the third picture, for light protection, is it very light-tight? If too much light is getting through either at the cutouts or on the perimeter, it could indeed cause the problem you are having.

I would still tell you to add an active form of aeration for your nutrient solution. My best guess on your problem is inadequate oxygenation of the roots. Fork out the $10 for a pump at wally-world and another $4 for some air stones. Put those in your reservoir and let them run 24/7.

Another thing you might try, if you have any more plants not attached to the nutrient film, is to remove one plant and set it on a plate out of the system. Water it a couple of times a day to keep the rockwool moist and see if the roots do any better.
gainesvillegreen Reviewed by gainesvillegreen on . brown roots? but not root rot!!! have nft.canna aqua vega 1.2 cf, ph 5.8, tap water ( been tested ok ). about a week after being put in the tank the roots under rockwool block turn a sort of translucent brown and are weak, but do not go mushy, roots outside of block are white but eventually go the same. plants still take up water, but growth is very slow. stem go's purple and older leaves, then others yellow suggesting slow,or no neut up take. everything is always sterilised and rinsed, but this has happened 3 times in a row. Rating: 5