Fish oil supplementation is, like most other supplementation, really only applauded by the supplement industry.
There's no proof any of it does much, which is why here in the UK all supplement advertising has to include disclaimers like "May benefit the elderly or those on a restricted diet"
There have been some decent clinical trials showing a high does of fish oil (something like 1g a day) improves mood better than placebo in bipolar depresssives. There are some risks to taking really high doses of fish oil every day though - and not just mercury content.
Similar trials for just doing plenty of regular excersise such as walking show a better result. Given that excersise is really good for you, it's probably a better option.
There's no proof any of it does much, which is why here in the UK all supplement advertising has to include disclaimers like "May benefit the elderly or those on a restricted diet"