Why the 't' ending when fish is common gender, but not for 'rød'?
Because it's an adverb here (like "completely" in English), which (when formed from adjectives) tend to take the "-t" ending, whereas "rød" is an adjective directly describing the fish
No, it is not wrong. "fuldstændigt" describes the adjective, is thus an adverb and doesn't relate to the noun, so it doesn't have to agree with the noun, whereas the adjective is related to the noun and has be in agreement with it.
Why can't we use there "hel"?
I'm a native speaker! You should be able to use helt.
It should be helt.
I used helt and it doesn't work either. Odd
Det er et biord, der beskriver tillægsordet, derfor : fuldstændigt (eller helt eller totalt)..