The previously posted information is incorrect, and is for the Yanxian Koi, as Kara Njara noted.
Correct Killifish information is as follows:
Location: https://www.xivdb.com/placename/2819/nem+khaal - https://www.xivdb.com/placename/2411/the+azim+steppe (X:32 Y:30)
This method works great!
Sold by Grand Company Quartermaster for 200 company seals. (Sergeant Third Class, Materiel)