In between his feature films, Tsai has always liked doing documentaries too. They tend to be somewhat rough and unpolished, but they're early indicators of the direction Tsai would come to take in more recent years. Fish, Underground perfectly fits that mold, so it wasn't really for me.
Without much background information or context, this isn't really a very coherent doc. It's broadly divided into three equal parts, events or scenes that Tsai simply captures on camera. The middle part, with the dead fish in the ground, is the most captivating one, though only in a very abstract way.
The footage too rough for what could've been an atmospheric slice of life, the lack of coherence is too grand for any obvious themes to transpire. What you get is Tsai's lingering camera focusing on three scenes he deemed interesting for some reason or another. I clearly prefer his feature films, this is a bit too meandering and amateurish for my taste.