The last time I used a daylight tank was in the mid '90s so maybe newer models are different, but I believe they are made out of black/dark plastic? Several right angle turns of black plastic provides multiple opportunities for a visible-light photon to be absorbed into a black body where it should be re-radiated as infrared (heat).

A hole/leak in the metal wall of a Faraday cage is going to be a lot more reflective and can easily act like a crude waveguide[1].

[1] https://en.wikipedia.org/wiki/Waveguide_%28radio_frequency%2...