Tetrahymeniasis is a ciliate disease that presents significant economic challenges for the aquaculture industry. Previous research has shown promising control effects on Tetrahymena pyriformis using flavonoids from Psoralea corylifolia (P. corylifolia), but their high cost hinders practical application. This study aims to find an affordable and safe alternative antiparasitic extract derived from P. corylifolia. Initially, six different solvent extracts from P. corylifolia were compared for in vitro antiparasitic activity and toxicity, with ethyl acetate (EtOAc) extract selected for in vivo testing. In vivo tests revealed that effective concentrations of the EtOAc extract approached toxic levels. Chemical analysis identified bakuchiol as the most abundant and toxic compound in these extracts, with the highest solubility in n-hexane. Two optimized extraction protocols, yielding extract I and extract II, were developed based on these findings. Extract II successfully reduced the level of bakuchiol and overall toxicity while maintaining efficacy. At a concentration of 40 mg/L for 24 h, extract II achieved a 100 % antiparasitic effect with a therapeutic index of 2.121, nearly double that before optimization, leading to an increase in survival rate of diseased guppies from 15 % to over 65 %. These results suggest that this bath therapy could be a practical and cost-effective alternative for treating tetrahymeniasis in aquaculture. Futhermore, the optimized extraction method employed in this study offers new possibilities for reducing toxicity and enhancing the effects of Chinese herbal medicine. In summary, this study demonstrated that extract II derived from P. corylifolia has the potential to be developed into a novel commercial drug for the control of Tetrahymena infections in fish.