You have to use bait, fish, I caught mine finally I think using a Koi in the grassy park at the central pond. The name escapes me.
I spent literally days fishing. I tried small fish bait, medium fish bait and large fish bait. I had two sims fishing at once, one with a 10 fishing and one with a 7 fishing. I can't remember which one caught it I was just too excited as it was the last fish I needed for literally weeks. The probability is just very low, and bait gets consumed very quickly. And random is unfortunately random.
I moved all the fish of various types between each fisherperson so that they had more bait between them and could fish longer with each type. As they ran out I changed type (eg one would fish with treefish, one would fish with minnow, when that ran out I just moved to another type of fish). I fished for days so I had a lot of bait accumulated.
I just finally got lucky with Koi.
I had better luck with space rocks and alien fossilized heads :P
