covering ebb and flow tray with plastic... good idea?

I was told to cover my 4x4 ebb and flow tray with plastic to hold in moisture and keep light from hitting the roots. The plants are in 4x4 rockwool sitting on top of coco mat. A friend told me this may cause the roots to not get enough oxygen... any thoughts on this? Would like to keep the plastic on because it helps keep the humidity in range but all about maximum happiness/efficiency here.

I use Panda film (bl/white -white side up) to cover my table. As far as oxygen goes I use an air stone and my return is high enough to to use the water fall method to add new oxygen.