Attiland
Aquarium Advice Apprentice
I'm actually not sure if the news story I read was legit or not. Since I can't read Swedish it was difficult to try and verify. I did find it mentioned in two sources, was about a year or two ago, but when I tried to find the articles again recently I was unable.
My theory was that fish will be afraid of what might attack them through the glass, but a solid background gives them a visually safe place they know they can't be attacked from.
Actually it is true. Due to similar law it is illegal to sell fish bowls in Hungary so Sweden is not the only one.