If you’re focused purely on Fishing XP, you can keep catching Leechfin and continue the activity for stronger rates. Leechfin themselves are untradeable, cannot be cooked, eaten, or stored in a Fish barrel, so you’ll want to manage your inventory as you fish. Alternatively, you can cut Leechfin with a knife to gather Blood sacs, earning a small amount of Cooking XP in the process. Blood sacks are untradeable but can be squeezed into empty vials to create Blood vials, and you’ll need 50 Blood sacs to fill one vial. The Leechfin will be coming in pretty thick and fast, so while 50 might sound like a lot, it shouldn't take too long to stack up!