Plant wilting severly during dark period...

I am growing two separate plants in two DWC buckets. Both are in the same conditions. One is extremely healthy but the other is not doing as well.

They are still in veg but when in the dark period, the struggling plant leavves wilt severely. The leaves perk back up once the light comes on. What could be the problem here? The leaves on this plant display a little curling even when the lights are back on. No burn or spots appear on the leaves.
 
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