Synthetic silver nanoparticles (AgNPs) are recognized as an important material with an unquestionable antimicrobial activity. Recently, biological AgNPs synthesis has gained more attention by the researchers due to being a simple, low-cost and eco-friendly method. Although, until now, a few studies have been conducted to evaluate their antimicrobial activity and the potential toxic effects where this nanomaterial has been exposed. In this context, our study focused on evaluating the antifungal capability of the AgNPs from mangrove fungi Aspergillus niger IB-CLP20 and their toxic effect on the routine metabolism of model organism Danio rerio. Antifungal assays were conducted by minimum inhibitory concentration (MIC) tests exposing several pathogenic fungi to the AgNPs. To assess the potential toxic effects of AgNPs on specific oxygen consumption, D. rerio were exposed to different concentrations of biological AgNPs. As controls AgNO3 and commercial AgNP were used. Results obtained by MIC showed antifungal activity for all strains analyzed, MICs ranged from 20 to 75 μg·mL-1. In addition, mortality was not detected for Danio rerio after 24h of exposion in a concentration limit of 100 μg·L-1. The results obtained in this study indicate that AgNP can be a promising option for many industries and the environment.