Water Conditioners & Supplement should be used with freshwater aquariums only. This is because the water conditioner does not remove chloramine and heavy metals. Instead, it neutralizes them, rendering them harmless to your fish.
You use a water conditioner every time you change the water in your tank or top it off. Typically you do partial water changes once a week for good quality freshwater. If your fish are sick or your water parameters are not right, you change the water daily and add a conditioner at that time.