Crocokoi

Crocokoi is a Limited fish tied to the Animal Event in the Second Sea, and the entry marks it both as limited and as obtainable, so it can be fished while the event runs. It bites on Minnow bait, at any time of day, in Foggy weather during Autumn. The text describes a scaly swimmer with snapping jaws that lurks in the shallows.

It is heavy for its price: 500 to 2,940 kg, averaging 950 kg, at only C$0.5 per kg. That gives an average sale of C$475 and 1,500 base XP. The average value ranks 401 of 742 documented fish, a little below the middle, but the max weight ranks 130 of 663, so it is firmly in the heavy end.

The Animal Event also lists Capybass, Cluckfin, Duckfin Tuna, Krabbit, Kittyfish and Minnowse at the same rarity, a full set of animal-themed catches.

The XP is the reason to go. 1,500 base XP is considerable next to a C$475 sale, so use Crocokoi to progress rather than to earn. Check that the event is live before you load Minnow, and remember that Foggy Autumn conditions are part of the requirement.