I'll throw my 2cents in, for what its worth coming from a more scientific side with little experience. Take it with a grain of salt.
My field of expertise is cell and molecular biology, and I'm a research scientist for a biopharm company. I focus primarily on small proteins and interaction....so not with fish.
In my job, difficulties constantly arise when trying to work with these and the best way I've always overcome them is to "think like the protein". So I will "think like the fish".
As the fish, drastically reducing the total volume (lets say greater than 30%, most normally found during acclimation period or when getting lazy and doing 50% change when the fish are already stressed from being in a new environment/establishing territory) of water would be a stressful experience due to the closing-in feeling, especially in tanks with large amounts of vegetation or more importantly lots of fish. Those invisible boundaries that many fish spend so much time setting up and "protecting" are dissapearing. Top dwellers have all but lost their space, and bottom dwellers have suddenly got a lot of playmates they might otherwise have tried to avoid.
Now compound this with the addition of water that "tastes" differently (due to lost salts, chlorine, or byproducts of the dechloronation process).
Now compound that with the possible temperature difference of the newly added water. While I feel the closer proximity of fish will cause a lot of stress, I would imagine as a fish the temp change would be cause for the most stress (if not temp matched before addition). Remember 2 things about fish, 1 they are tiny compared to humans, so their surface area to volume ratio is greatly increased. Imagine putting your bare hand out a car window in the dead of winter while your driving. Now compare that to sitting on the roof of the same car NAKED. That's a completely non-scientific analogy to how quickly heat changes occur in small animals, but it serves the purpose.
And 2, we are dealing with water not air. Water has the heat capacity of something like 600times that of air (don't quote me
). That means it can add or leech heat from an object MUCH faster than that of air. Think of when someone flushes the toilet when your taking a hot shower. That's shock, and while it may not cause you to go into cardiac arrest, imagine if you had the same surface area to volume ratio as a fish!? That would be quite stressful.
So like anything its a balancing act. Balancing the stress of everything mentioned above from water removal, with the benefit of removing contaminants from the water. What's the best choice? Probably a compromise between the 2. Once my tank is up and running I'll be doing
PWC's of max 25% with the same temperature of water (probably aiming for 1-2 degrees above tank temp to account for the heat lost during syphoning back into the tank at a slow rate), but instead of dumping a bucket of new water in, or removing a bucket full in a short time, I'll set up some type of drip-syphon so no water gets added/taken out too quickly. I think doing something like this will limit the amount of stress on the fish. But who knows...
justin