Mammal heart is recommended by some for specific reasons, but generally I think the above answers it all. Deer is too tasty for fish and the fish aren't designed to process mammal flesh.
I highly advise against feeding fish organs, especially liver. Even humans (who have eaten deer livers for thousands of generations) suffer medically from eating it. For us, eating liver once a week is probably far too much, so I think that means never for fish.