Where do you have the year from? The aquarium was opened 2003 after several years of building. And seeing it now, two weeks ago it even had its 19th "birthday". Speak about bad timing.
Not sure about Germany but in the UK the designers would be unlikely to still be liable anymore under the original contract to design this, liability under the contract would be up to 12 years from when the work under the contract was completed. They might be liable in tort for professional negligence but it's much harder to prove that this has occurred especially after 20 years. More likely the company that was under contract to inspect/maintain this will get hammered.
ICM has already removed AquaDom from their portfolio page https://icm-corp.com/portfolio/ , but a cached version (for now at least) exists at https://webcache.googleusercontent.com/search?q=cache:XGSNC7... .