[SantaMonica]

Neat. The black walls probably make it think it's in an open body of water, and thus can escape if needed. Clear or other color walls show things nearby and thus no escape. So it stresses.

Blue ribbon eels are similar. If they don't have a long pipe to hide in, they will know that their body can be seen and thus eaten, because there is nowhere to hide. So they stress and don't eat, and starve.