Microplastics are a complex suite of contaminants varying in size, shape, polymer, and associated chemicals, and are sometimes referred to as a multiple stressor. Still, the majority of studies testing hypotheses about their effects use commercially bought microplastics of a uniform size, shape, and type. In this study, we investigated the effects of pre-consumer polyethylene (PE), pre-consumer polypropylene (PP), and a mixture of PE and PP collected from the shoreline of Lake Ontario, where they likely sorbed a complex cocktail of contaminants. Embryo-stage fathead minnows were exposed to physical plastic particles or chemical leachates alone at an environmentally relevant (280 particles/L) or high (2,800 particles/L) concentration for 14 days. We have shown that the effects of microplastics differ by polymer type and presence of environmental contaminants, and that effects can be driven by the physical particles and/or the chemical leachates alone. Larvae exposed to pre-consumer PE experienced a decrease in survival, length, and weight, while pre-consumer PP caused an increase in weight. Environmental microplastics caused a more drastic increase in length and weight, and almost 6x more deformities than either of the pre-consumer microplastics. While pre-consumer microplastics caused effects in the physical treatment only, the environmental microplastics caused effects in both the physical and chemical treatments, suggesting that the mechanism of impacts is context dependent. This study provides further support for treating microplastics as a multiple stressor and begs further research on the effects of environmentally relevant concentrations of microplastics on the long-term growth, survival, and reproductive output of aquatic organisms.