I've also been researching fish oil. So far, I've read that there can be "too much? of a good thing on some levels in humans - causing vision problems etc.
Also, with anything "fish" condensed - Mercury is a concern I have. I'm having trouble finding anything really - that addresses either of these two issues with supplementing a dog's diet with fish oil. I can't start giving this on a regular basis until I know there are no long term "more harm than good effects".