Fish can be sensitive to an extremely wide range of environmental conditions. These can include nitrates, nitrites, water hardness, chlorine, pH, alkalinity, sunlight, algae levels and of course temperature.
Educate yourself on proper conditions for each species of fish in your aquarium. Some fish are highly sensitive to conditions, and some species may require very specific parameters. So make sure to do your homework. Once you know the correct conditions, monitor and maintain them closely.