I partially disagree with this comment. While we should certainly avoid extremes we also have to recognize that many fishes are tank bred and the condition in which they were raised is quite different than that found naturally. If they were suddenly thrust into conditions found in their native habitat one question if they would rapidly perish. Furthermore many fishes live under the conditions they live because they have no choice not because it is the optimal environment. Having said that clean water is certainly likely to be more advantageous than polluted water or water with heavy doses of chemicals; but the particular with regards to ph,gh,kh are not likely to be those found in their natural habitat.