The impact on aquatic life and on humans can’t be compared directly. First of all, toxicity is often related to body mass, i.e. the lethal dose is approximately

The impact on aquatic life and on humans can’t be compared directly. First of all, toxicity is often related to body mass, i.e. the lethal dose is approximately proportional to the weight. This means, that a lot more of a substance is needed to have the same effect on a human than for a tiny fish. Second, and perhaps more importantly, aquatic animals are immersed in water and they are so from the very beginning. While we drink for water intake, fish “breathe water”, i.e. they constantly have water flowing into and out of their body. This makes them more susceptible to contaminants in water.