The most amazing thing about this episode was that they kept calling that mudskipper a mole for the whole episode. I'd find it surprising if the Japanese had the same word (mogura) for a fish and a mammal, but maybe there's some explanation for that.
Having a filler here doesn't really surprise me. Even in the manga they indicated the folks performed missions between the tournament arc and the next one, even if they didn't really show them. They could have made the mission a bit more glorious than this, though. They just won the tournament and got declared the most powerful mage guild, pushed back dragons, and now they barely defeated a fricking fish. Had this been One Piece, that fish would have been roasted and being eaten one minute after they first saw it...