Two randomized placebo-controlled trials found that supplementation with omega-3 fatty acids (1.2 g/day) improved mood, cognitive functioning and reduced psychosis in high-risk adolescents compared to placebo.

The finding that treatment with fish oil may prevent or at least delay the onset of psychotic disorder gives hope that there may be alternatives to antipsychotics for the first psychotic episode. Omega-3 fatty acids have been shown to be very safe even when used in relatively high doses, and except for gastrointestinal symptoms like fishy eructation, they are free of clinically relevant adverse effects. They have the advantage of excellent tolerability, public acceptance, relatively low costs, and benefits for general health.

Second study: http://journals.sagepub.com/doi/10.1177/070674371305800705