The only time that I use a dechlorinator is when filling a pond for the first time or when refilling after a complete clean-out. Since I don't advocate water changes, that scenario is never presented. There are several reasons that I follow this practice-
-Chlorine dissipates quite rapidly when exposed to sunlight. (difference between a pond and an aquarium).
-Most municipal water suppliers use different levels of Chlorine depending on need. These levels are normally considerably below the LC50 toxicity levels for Carp and Goldfish. These suppliers will at times "Shock' the system with very high doses but this is usually done in the wee hours of the morning to minimize human contact.
-No where can it be found exactly what concentration of Chlorine a dechlorinator is effective on. As a result, even the recommended dosage found on the label is a 'shot in the dark' as to effectiveness.
-Any Chlorine that is added to the pond, while already in dilution, will be diluted even moreso, greatly reducing the mg/L readings and toxicity.
-There has been some evidence presented that Sodium Thiosulfate, the usual compound used in dechlorinators, can be an irritant to fish resulting in overproduction of mucous.