Thought maybe a picture would assist in this discussion.
Representation is an interesting thing. I don't believe it was meant to be modeled after something physical, like a fish as you suggest, but more to use imagery to provoke subconcious ideas and emotions about the character. This goes for all character design, but the face and head are what we as humans focus on the most and draw the majority of our first conclusions from. With Zant, this happens to be his mask, because until the end of the game, this is all we see.
Focussing on Zant's mask, in physical appearance the mask resembles a snake or lizard head (at least to my eyes). Looking at that from a symbolic perspective, this could related to underlying characterization of Zant as 'the evil one' and cunning like the theological snake. But also, it appears to be something unreal and undefined, bringing in another sense of unease. These two subconcious portions of the design, as well as the grey/silver scale of the metal, are all involved with making him appear as the villain.
Height is another factor we see in the structure, usually representation of prestigue. This is even seen in the first cut scene we see of Zant from the downward angle with two shorter minions beside him. This makes for a very good show of power. The leader of the villain, or in Zant's case an attempt at it. but we only know that from the plot not from the design.
It also creates and air of mystery. We never see Zant's face (until the end of course). We know we are dealing with this villain, but we cannot see him. The game designers even tease the player's mind in Lanayru Spring when the bottom portion of the mask retracts and we can see just Zant's terrifying mouth. Mystery and fear/unease are emotions an audience should feel form a good villain, and the creators used the mask design to do this.
The really interesting part is the similarities of the eye shape to that of the fused shadow and Majora's mask. In some ways, this really shouldn't come as any surprise. We know that Zant is a Twili and we know the fused shadow was part of that race as well. As for Majora, that is a theory for a different thread. But I will touch just the tip, that Majora's mask may have history with the dark tribe/ancient tribe. This has no real significance on the emotional impact or representation of the mask, but it does tie Zant into the tribe in a visual sense.
Overall, the mask is supposed to represent the villainy and create mystery. i don't believe it is meant to represent anything physical, but may hint at the ties to the dark tribe we have heard about with the fused shadow and Majora.