Leather Carp (its a yellow fish) can be caught while afk fishing in Tarif (anywhere near the fisher npc, if you need more specific spot). No need to run around the Demi River in Heidel and look for hot spots.

Поймала на рыбном косяке на реке Деми, немного западнее Хиделя. Ловила на золотую удочку с энергией. Косяк исчезает после первого заброса, но можно сделать еще 1-2. В итоге за 2 заброса поймала 5 Гуппи. P.S. На рыбных косяках можно ловить только с лодки, с берега не прокатит.

If you haven't gotten to Tarif yet (I hadn't - was a new player), get a raft or rowboat to Torphin (Otter fishing NPC on Demi River west of Heidel). Roam around east and west of Torphin and you'll hopefully eventually see schools of fish jumping out of the water. These hotspots don't last very long, but will get you Leather Carp. You can continue fishing where a hotspot used to be for about 3-5 minutes before you'll stop getting gold fish there. It took me about 4 hours of searching on the river to finish this quest.

All you actually need to do here is stand at the pier in Heidel and fish. Since it's a yellow grade fish it will take some time to get five of them but you can hand them in one by one at the NPC Grace Lauren in Heidel.

with the change from the quest requiring blue grade instead of yellow grade fish, Tyccha's method of going to Torphin still seems to work (without having to actually fish from a school) since Heidel's resources are almost always exhausted.