I have no experience in creating sprites for Zoo Tycoon, but here's an idea I had for a mod that I wanted to pitch.
I've heard often that the bluefin tuna is REALLY underwhelming, particularly in size. So here's something I thought about; I took notice of Borsato's Raptor Patch, which gives the in-game Velociraptor new ZT2-based sprites, then reuses the original sprites for a new animal Deinonychus.
Perhaps something of similar sorts could happen with the tuna, giving it a new set of sprites that's much larger, more impressive-looking, and more accurate, then using the original sprites for some other kind of fish(admittedly with the sprite being really small it looks really nondescript, so it could be anything).