General terms:
* Slender: This is a general term for a fish with a long, thin body.
* Elongated: Similar to slender, this emphasizes the length of the fish.
* Lean: This suggests a fish with little fat and a streamlined body.
* Streamlined: This specifically refers to a body shape that minimizes water resistance.
Specific terms:
* Ribbon fish: Refers to very long, thin fish, like the Regalecus glesne (oarfish).
* Pike: Pike are known for their long, slender bodies and predatory habits.
* Barracuda: These fish are also long and slender, and are known for their sharp teeth.
Figurative terms:
* Svelte: This implies grace and elegance, which might fit a particularly slim fish.
* Willowy: This emphasizes a delicate, slender form.
Ultimately, the best term to use depends on the specific fish and the context in which you're using it.