Something I forgot to mention in my post last night, but I read that vit. C neutralizes chlorine faster then sodium thiosulfate. Sodium thiosulfate is a major ingredient in many dechlorinators. I myself bought 1 pound of the dry stuff to see if would work on my city water. I've had it for months, but haven't gotten around to testing it yet lol. The issue with using just sodium thiosulfate and I would assume(for what I've read) its the same with vitamin C, is if you have chloramines. If chloramines are present it does only half the job. The compound chloramine is basically a ammonia ion and chlorine ion combined in the form of NH2CL. This stabilizes the chlorine ion so it does not simply evaporate like it would in the free state. It also brings up the problem of using conventional 'simple' dechlorinators, if you neutralize chloramine with just sodium thiosulfate the result is you release that ammonia ion. So now the chloramine is gone, but essentially their is an equal concentration of ammonia now present. Ammonia being toxic as well, many dechlors these days have added ingredients to effectively minimize the ammonia toxicity until the biofilter deals with it. Basically it comes down to which is more toxic chloramines or ammonia and that gets much more complicated since ammonia's toxicity is related to pH.
To be fair the chlorine/chloramine concentration in MOST taps is very low. These additives cost money and there is no reason to add more then required to reach the farthest consumer. Many dechlorinators go seriously overkill. For example if you just have chlorine Prime at recommended dosage treats 3.3ppm, which is more chlorine then you will find in most swimming pools. Many people do partial water changes without any dechlor as the low levels in most tap water gets further diluted by the tank water and then will be exhausted quickly in a place that is so biologically active.
You could test if you have chloramines and their rough concentration buy filling up a bathtub and treating it with sodium thiosulfate. If an ammonia test shortly after reveals ammonia where there was none before it would highly suggest you have chloramines. However its not safe to conclude you have chlorine if no ammonia is present. You could also test Vit. C this way if you know you have chloramines.
In an emergency neutralizing tap water is a headache regardless. Given if they have no dechlor I doubt they have a test kit handy. IMO it would be better off just mixing tap with natural sources of water. Ideally rain water or melted snow if the tap water is really whats killing the fish. Most fish will survive in fully untreated tap water, its not good for them, but short term its often not deadly.
