It seems the high level you get, the less the battles drop. Even bosses often drop nothing. Just the nature of the game.
Don't say these pessimistic things , you will see... just an example: Friar Sand in Moshuu can drop every time you beat him (take 5 minutes) around the same amount of gold you get from captain Gunn treasure , not mentioning the epic gear too.
Lava fishes are just on the passage, stomp on them and pass over .
The problem, I think, is that treasure chests appear where the enemy died. In that particular fight, there is a lot of open air where the fish can go but you cannot. So if you kill them out there and not on the bridge you won't get any treasure.