Hi Wilfie. I have read an interesting explanation for what you are talking about.
The author of the book I have read calls it "Dark Ages" (lol). Well, here is what he says. In his own words: "Back in the old days the common wisdom was that the older the water in your tank, the better. The only reason fish even survived is that by having old water and undisturbed tanks, we also tended to have functioning biofilters on the plants and in the gravel without knowing it. If we set up a new tank and moved fish into it, they often died. You see - 'new tank kills'! This way fish have adopted slowly to increasing quantities of pollutants, particularly nitrates, that accumulated in the never-changed tanks, so when we greatly freshened their water it was an enormous shock, even though the water was vastly superior. At the same time, the new tank had no biofilter established, so ammonia began to accumulate the instant the fish were added. I am telling you this to demonstrate that I once believed that water changes were unnecessary, even bad. What changed my mind? Water changes! I found out that when I paid attention to biofiltration and performed lots of water changes to remove nitrates and other pollutants, my fish thrived as they never had before. I was spawning fish I'd never been able to keep before. But what really convinced me was the fish themselves and their reaction to water changes - they love them!" (David E. Boruchowitz, in "The Simple Guide to Freshwater Aquariums")
I hope that helps
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