there's two covers. there is a black one under the white one which has been cut in this way X so that each corner sits on top of the block and stops any light getting to the roots. you can just see it on the third pic. the white one is purely for reflection . i have put an extra pump in the tank which just feeds back in resovior near the main pump. it makes a lot of splashing noise though!!!!! all the plants i have taken out of the tank and stripped of brown roots are all rooting again, so next time i intend on only running the solution once or twice a day.