I would have to disagree with a number of points in that article.
1. Conductivity, while important, is not the single most important factor of keeping aquariums. And his comments on the ratios of ions in water is incorrect. In the Rio Negro the concentration of sodium to magnesium or calcium are equal to or more than twice as high. Not lower than magnesium or calcium as he stated.
2. Conductivity can vary widely in the short term such as after a precipitation event and fish are able to adjust to it rapidly. Fish have kidneys that allow them to adjust their osmotic pressure just like humans do.
3. pH shock definitely does exist. Non-toxic ammonium becomes toxic ammonia at alkaline pH values. Suddenly increasing pH can lead to a rapid increase in toxic ammonia in the water. In addition a low pH can lower the oxygen carrying capacity of the blood. Suddenly taking a fish from a high pH and putting them in a low pH can basically lead to suffocation as the blood pH drops.
And lastly Cortisol measurements as a stress tester (although an industry standard) are not always accurate and can be influenced by unknown variables that will affect results.
For more information:
http://www.fao.org/docrep/003/T0537E/T0537E02.htm
http://www.panamjas.org/pdf_artigos/PANAMJAS_4(2)_158-178.pdf
http://www.ca.uky.edu/wkrec/interactionsphetc.pdf
And there are really only two big manufacturers UPG and Central Aquatics. Royal has their own product line (Deep Blue) but is still mostly a distributor.
And of course they make products for their marketability, why else would any business do it?
Andy
