Psilocybin given through water immersion at 3,000 µg/L for 20 minutes significantly reduced movement and aggressive swimming bursts in mangrove rivulus fish (Kryptolebias marmoratus), with 16 fish per treatment group. Treated fish moved less than controls with a large effect size and showed fewer aggressive bursts with a medium effect size, while whole-body psilocin levels reached about 7 ng/mL, near the 11 ng/mL peak reported after a low-to-moderate human dose. This is a weird but useful model story because it shows psilocybin calming social aggression in a naturally aggressive, self-fertilizing fish.