"Not just put him into a tank of water like that! He will have nothing to play with. That's why the kid is stressed. "
This is not really true, tons and tons of people have bare tanks including myself. My flowerhorn went from sand and lavarock, to red/black gravel and plants, to bare tank with a handfull of gravel to spit around. All the while never showing any signs of stress, whatsoever. Healthy appetite, no bars, no hiding, just a happy energetic little guy no matter what i change, though i try not to change things too quickly/dramatically just a little at a time.
I would however change that background back to blue like he had before, that may be what brought it on. I have never changed the blue background i have its bright and he seems to be happy with it.
See here he is, happy as a pig in

in his bare tank. Video of
My Flowerhorn