Swellfish is actually named Angler

Now it is Siganid. You can catch them standing next to quest giver (or anywhere in Balenos Islands area).

With harpoon it's easier to catch them

แนะนำให้จับโดยใช้ฉมวกๆ จะได้ง่ายมากๆ

Since the implementation that every old White fish is green, and green is Blue, its close to impossible to fish the "Rabbit Fish" / "Siganid" at the Quest NPC. spend over 10 hours there, not a single blue fish!

-> Fished near Lema island, and got "Rabbit Fish" after 2nd try.

FR : Le sigan est trouvable ici.
EN/US : Angler is available here