How would you know if a dissolved molecular substance caused the fish kill?

To determine if a dissolved molecular substance caused a fish kill, one would analyze water samples from the affected area for the presence of toxic compounds and compare them to control samples from unaffected areas. Additionally, examining the health and tissue samples of the deceased fish for signs of contamination or exposure to specific toxins could provide evidence. Investigating environmental factors, such as changes in water temperature or oxygen levels, would also help establish a causal link. Correlating the timing of the substance's introduction with the fish kill event is crucial for establishing causation.