Micro- and nanoplastics derived from environmental degradation of larger plastic debris can be ingested and accumulate in aquatic organisms, raising growing global ecological concerns. Toxicology studies of aquatic organisms predominantly use commercial formulations of micro- and nanosized polystyrene particles as model plastics. However, many of these commercially available formulations contain different preservatives, antimicrobials, or surfactants such as sodium azide, Tween 20, and sodium dodecyl sulfate, which may introduce artifacts in toxicity assessments. In this study, we carried out acute toxicity tests on Daphnia magna, using commercial 20 and 200 nm polystyrene nanoparticles (PS-NPs) containing 2 mM sodium azide as an antimicrobial preservative. The acute toxicities of nondialyzed PS-NPs, dialyzed PS-NPs, and sodium azide alone were compared. The results reveal that the acute toxicity of the complete commercial formulation of PS-NPs was mainly associated with sodium azide and not the particles themselves. The dialyzed PS-NPs did not cause mortality but significantly disrupted the swimming behavior of D. magna. As commercial PS-NPs are commonly and increasingly used in plastic toxicity assessments, these results highlight the importance of considering the impacts of the suspension matrix.