Acceptable concentration 0

A gas widely used in the disinfection of water and as an oxidizing agent for organic matter, manganese, iron, and hydrogen sulfide.

Chlorine is known to react with organic matter in the water to form trihalomethanes (THMs), a suspected carcinogen.

Make a solution consisting of 4 ounces (1/4 lb) Sodium Thiosulfate crystals (photo or technical grade) dissolved in 1 gallon of distilled or deionized water. Use 5 ml (1 teaspoon) of the solution for each 10 gallons of makeup water to neutralize up to 3.75 ppm chlorine.  One cup can be used for each 500 gallons. (The entire one gallon of solution will treat about 7500 gallons of tap water.) The shelf life of the solution is about six months when stored in a cool location. The crystals will keep for several years if kept dry.

NEVER use chlorinated tap water to clean your bio converter (filter) media unless you are actually trying to sterilize it.  Water from the pond is a much better choice for this task.