I have never heard of anyone feeding there fish pig heart. I would think that if it was heather it wont be by much. But I still think beef heart would be safer.
the heart being a muscle would contain very little fat. You should still trim it properly just like beefheart but it will work just the same. Alot of online fish recipes use the 2 (beef and pig heart) interchangeably which makes me think they are fairly similar.