Many discus breeders have very little effective biological filtration since their tanks are at a low enough pH that bacterial metabolism is inhibited and ammonia and nitrites never get broken down into nitrates. That's why they have to rely on constant water changes to keep these factors in check. Since the bacteria aren't doing much in the first place in these setups, they don't have to worry about large WCs negatively affecting the bacterial loads.
That being said, I agree with Pufferpunk - large water changes are not really a problem if they are done properly.
Ok Xander, here goes:
Theoreticaly, 100% water changes are not a problem IF the new water is identical as far as temperature, pH and chemical composition goes and they are done regularly. The reason that they usually are a problem is that very few people can really keep all these factors constant between their tank and the tap. So the larger the water change, the greater the probability that you're going to have a swing in one of these factors.
Differences in pH will alter the toxicity of ammonia (among other things), will alter the metabolism of nitrifying bacteria, and can also influence the pH of the fish's blood which will cause stress. Differences in temperature will alter the solubility of oxygen in water. Both temperature and pH will influence the efficiency with which hemoglobin in the fish's blood can bind and release oxygen (if interested, look up the Bohr effect).
Even if large water changes are done properly, controlling for the above factors, constant large water changes will eventually result in a reduced nitrifying bacterial populations since you are artificially lowering their food sources, but as long as you keep up the water changes that's fine. If you suddenly stop doing the water changes bioload will probably increase faster than the bacteria can re-establish themselves and you'll end up with an ammonia/nitrite spike.
Detailed enough for you?
