Starlit Seahorse

A fish that tops out at 0.2 kg can still be Legendary: Starlit Seahorse ranges from 0.1 kg to 0.2 kg with a 0.15 kg average, which is rank 1,037 of 1,059 by maximum weight. What it lacks in size it makes up for in price per kilogram, listed at C$13,250.

At the average weight that gives C$1,988, ranked 365 of 1,038, with 1,850 base XP. The wiki lists Astral Observatory as the location, Seaweed as bait, and Night, Clear weather and Winter as the conditions, so every condition field is filled in. Its quote calls it "a tiny syngnathid whose dark body is speckled with pinpricks of brilliant glowing starlight."

Three other Legendary fish share the zone, Moon Jellyfish, Riftia and Leopard Shark, out of 35 documented in total. The status line lists it as obtainable, and it is not marked as limited.

Verdict: the Seaweed, Night, Clear and Winter combination is a long list, so check all four before planning around it.