Low dissolved oxygen and high unionized ammonia are two common environmental problems that can sicken or even kill fishes.
Water quality parameters should ALWAYS be tested if fish are sick.
The first thing to check when fish become sick is the water. It may appear clear and odorless, but that doesn't mean something is not amiss. Basic water quality testing should include these parameters:
Dissolved oxygen (DO)
Percent oxygen saturation
Carbon dioxide (CO2)
Total ammonia nitrogen (TAN)
Nitrite ion (NO2-)