Without a doubt, fish oil has proven to be one of the most important source of omega 3's for our diets (ours and our pets).
While flaxseed also has omega 3's, its profile and digestibility are not the same.
Here's a snapshot of an excerpt by Barteges et al 2019 that I read this weekend:
This report cites 7 research studies in this snapshot alone!
Still not convinced?
Being the avid devourer of research and anything evidence-based for treatment, I have amassed hundreds of studies on various health topics and file folders devoted to alternative-type information (like fish oil).