@Shinigamisama It starts with someone trying to subvert expectations. But then other authors read that and think, "that's clever...I'll do it too!". Then before long the subversion is the trope...
In the first chapters, it was said that fish contains alot of mana so it might be the reason why the king is drinking the disgusting thing. Some con lured the king into thinking that the juice will extend his life.
@Mizerie Historically speaking even the Romans drank Garum, which was tantamount to fish-juice anyway, so I wouldn't be surprised if the guy really just liked fish juice. Though you're probably right in that it's likely some dubious plot point of his.
I do find it kind of baffling how the MC keeps saying he isn't strong when he has basically ended every fight in one blow and was able to manhandle the lolidragons dragon form with one hand. It doesn't quite add up is all I am saying.