Zinc oxide (ZnO) is one of the high-volume production nanoparticles (NPs) currently used in a wide range of consumer and industrial goods. The inevitable seepage into environmental matrices and the photoactive nature of ZnO NPs warrants hazard profiling under environmentally related conditions. In this paper, the influence of simulated solar light (SSL) on dissolution behaviour and phototoxicity of ZnO NPs was studied using a combinatorial library of ZnO NPs with different sizes, surface coatings, dopant chemistry, and aspect ratios in a fish cell line (BF2) and zebrafish embryos. Generally, the cytotoxicity and embryo mortality increased when exposed concomitantly to SSL and ZnO NPs. The increase in toxic potential of ZnO NPs during SSL exposure concurred with release of Zn ions and ROS generation. Surface modification of NPs with poly(methacrylic acid) (PMAA), silica or serum coating decreased toxicity and ZnO with serum coating was the only NP that had no significant effect on any of the cytotoxicity parameters when tested under both dark and SSL conditions. Results from our study show that exposure to light could increase the toxic potential of ZnO NPs to environmental lifeforms and mitigation of ZnO NP toxicity is possible through modifying the surface chemistry.