caz27 said:
Until u mentioned it anna i had never heard of it

so i think you should fill us all in cause i would like to know
Sorry, I just assumed it was common knowledge.
Like all animals, fish excrete excess hormones in their waste, along with ammonia (in humans our ammonia water is in the form of urea). These hormones are natural steroids and if they build up they can badly affect the fish's immune systems and reproductive systems. They can stunt the growth of fish and prevent effective breeding, and also lead to stunted and malformed eggs and fry. This may be one of the reasons that a water change is supposed to stimulate some fish to spawn. It may also be one reason why regular water changes have a good effect on fish's colour (since colour is often associated with breeding, which in turn is affected by hormone levels).
In the wild of course fish have a constantly renewing source of water, either from rain or from streams, so they don't have to deal with this problem and haven't evolved any defence against it.
Other chemicals that can build up include environmental toxins absorbed from the air (carbon monoxide, organophosphates, dioxins and other organic compounds). Although these are in the environment all the time, the warm water of a tank can "fix" them and levels can be far more concentrated then in the surrounding air. Although in minute quantities, a gradual build-up of these toxins is likely to have a bad cummalative effect on the fish's immune system, even leading to things like cancers, and potential shorten the fish's life.
Another issue is that the dissolved gases from the air can gradually displace oxygen and lead to a significant drop in the water's oxygen-carrying capacity.