Water companies are duty bound to provide water which is fit for human consumption. That’s it, no mention of fish at all.
In order to keep bacterial numbers down water companies apply disinfectant treatments to the water. In the UK this normally consists of chlorine, a bleaching agent. Chlorine is highly toxic to fish even at low levels (0.2ppm), to be fair if it wasn’t toxic it wouldn’t be a very good disinfectant.
Chlorine is unstable in water and it can be removed quickly and easily. If water is allowed to stand for 24 hours in an open container or better still vigorously aerated the chlorine content will simply evaporate.
Alternatively one of the many aquarium dechlorinators can be added to instantly neutralize the chlorine. Filtration through activated carbon, such as in a tap-water filter, is also very effective at removing chlorine.
Due to its instability and potential for forming toxic organic compounds the water companies looked for an alternative to chlorine. This search resulted in their use of chloramines, which are compounds of chlorine and ammonia. They are far more stable than chlorine alone and this makes our drinking water safer.
Unfortunately, it also means that they are far harder to remove from water for fishkeeping purposes. Simply aerating chloramine treated water or leaving it to stand will do pretty much nothing. Worse using a traditional dechlorinator will neutralize the chlorine component but release the ammonia component which causes its own problems. Thankfully there are specific water conditioners on the market which are able to neutralize chloramines completely. As time goes by it is likely that all aquarium water conditioners will have this ability. In the meantime if you live in an area with chloramines in the tap water check that your water conditioner of choice is able to remove them.
Chloramines are highly toxic to fish as they enter the bloodstream via the gills and reduce the blood's ability to carry oxygen in much the same way as carbon monoxide in humans.
Chloramines and RO's
Most RO units are fitted with activated carbon blocks in the pre-filtration stages, these are perfectly capable of removing both chlorine and to an extent chloramines. The aquarist should be aware that the removal capacity for chloramine is often far less than that for chlorine. For example our ultima blocks will treat 82,000 gallons of tapwater for chlorine reduction but only 10,000 gallons for chloramine reduction. Some carbons will only managed adequate chloramine reduction for a few hundred gallons. So if you have chloramines in your tapwater check how much water your carbon block pre-filters will treat and consider upgrading them.