Joe's spirit was then captured by Malefor, who reformatted the cleaner fish into a Dark Spawn Lord because Joe was apparently very cruel and malicious in life, while in reality, he was only as cruel as a moderate disney villain and very obscure as that no one had heard of him until after his death. Malefor then dubbed the fish as head of intelligence, and even had his henchmen made loyal to him again.
Of course, even though Joe looks nothing like a demon and has no Dark Spawn powers, he does have a special ability to transform into his super strong humanoid form when fighting on land. (insert picture of human joe here) aisde from that, he primarly sevres as a straigiest.
Behind the scenes, the reason why Scroopfan wanted to include Joe as a Darkspawn instead of any other demon is because Scroopfan is actually Danish-amarican and just learned from his Danish heritise from Help, I'm a Fish, and he was the only impressive thing (it was not bad, but could of been better) in the film, and thought he seemed very unhumanly cruel, (Dispite Cheirnibogfan's opinian to him as cruel as a moderate disney villain) that he wants to elevate the main villain into a demon under Malefor's servitude.