The same level of chlorine chemical that is safe for humans can easily be toxic and kill the fish in the aquarium. You have to use the water conditioner to help with eliminating the chlorine and chloramine that might be in the water.
Instructions for use:
Use 1 capful (10 ml) for every 100 gallons of tap or well water. For treating smaller amounts of water, we recommend using a graduated measuring device. The recommended dosage removes approximately 3 mg/L (ppm) chloramine or 4 mg/L(ppm) chlorine. To detoxify nitrite or ammonia in an emergency, it is possible to use up to 5 times normal dosage. If tap water is suspected to contain an excessive amount of chlorine or chloramine, we recommend up to a 5 times dosage.