While an aquarium contaminant is, technically, anything in the water, there are three major contaminants specific to aquariums:

• Ammonia -- lethal to most freshwater fish at 5ppm
• Nitrites -- lethal to most freshwater fish at 10ppm
• Nitrates -- not hazardous until 60ppm

These three chemicals are directly related to each other through a process known as the Nitrogen Cycle