Manufacturers of products such as these do not like to divulge their trade secrets, so it is difficult to determine the exact make-up without expensive laboratory analysis.
But one thing is certain. A common main ingredient in liquid dechlorinators is sodium thiosulphate (Na2S2O3).
When this is added to water containing chlorine (Cl2), they combine to make salt (NaCl) and a water-soluble form of sulphur called an anion (S2O3- -)
Na2S2O3 + Cl2 = 2NaCl + S2O3- -
(sodium thiosulphate + chlorine = salt + sulphur)
The salt and the sulphur at these low concentrations are harmless to koi and will be diluted by normal water changes or topping up after cleaning filters, but there are two side effects that you should be aware of :-
1/. Water companies often add ammonia to their chlorinated water to make chloramine, which is a more stable disinfectant for their purposes. So, if chloramine is present, then the sodium thiosulphate will only remove the chlorine and will leave the ammonia still in the water. The levels vary greatly from one company to another and according to what the companies think appropriate at the time, but in general, the level of residual ammonia will not be high. Under normal circumstances it will soon be removed by a properly working biological filter but you should be aware that if you are doing water changes to reduce the level of ammonia in your pond, a liquid dechlorinator may add to your problem.
2/. When sodium thiosulphate is added to water containing chlorine, they combine to make salt. Salt should not be added to a pond in which zeolite is being used to remove the ammonia as it will cause the ammonia to be released back into the water. (See zeolite below). Again the levels are not likely to be great but I advise caution when using a liquid dechlorinator in a pond where zeolite is present.