Questions & Answers about Själva akvariet är hälften så stort som vi tror.
How does the "hälften så ... som" structure translate literally?
Why is the adjective "stort" instead of "stor" in this comparison?
The adjective still needs to agree with the noun it describes, even when tucked inside a comparison phrase. Because ett akvarium is an ett word, the adjective takes the neuter -t ending to become stort.
Why is it "själva akvariet" and not "självt akvariet"?
When used directly before a definite noun to mean "the actual" or "the [noun] itself", this word acts like a definite adjective, so it takes the weak -a ending (själva). You would only use the neuter form självt if it came after the noun as a standalone pronoun (for example, Akvariet självt är... meaning "The aquarium itself is...").
Why is the definite form of "ett akvarium" spelled "akvariet" instead of "akvariumet"?
Swedish nouns that end in -ium (usually borrowed from Latin) drop the -um before adding the definite neuter suffix -et. This is why ett akvarium becomes akvariet, just like ett museum becomes museet.