So for what it's worth here's my experience -- generally not super impressed with supplements, tried cod-liver oil, didn't notice much effect, but after starting to make homemade salmon sashimi from costco salmon (Yes it's a thing, youtube it), I started noticing reduced anxiety and better concentration on days when I did so.

I would eat about 2/3rds of a pound of raw salmon and notice this effect (which would be like $60 or more at a sushi restaurant). I've done this at least 20 times now and never had any issues.

Generally I don't mention the mental effects of the salmon to people because I feel like they wouldn't take it very seriously, but since you're asking and it's on-topic, I thought I'd share.

Mackerel, sardine.

I believe they are amongst the best source of Omega 3. Technically you could still eat cod liver but you would still eat big amount of pollutants because it's the liver.