12-23-2009, 08:49 PM
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This is an interesting question, and I must say that in all my years of research I have not to the best of my recollection come across any mention of the effect of pressure changes during a partial water change. I do have a couple of related observations.
Fish can sense atmospheric pressure changes. When there is a low pressure system over the area, the fish know it; I doubt I would sense this if I did not look at the window and see clouds. It is often suggested that if a fish is difficult to spawn, do a major water change when there is a low pressure system overhead and you are all but guaranteed to have the fish spawn. It works very well with Corydoras and many characins. Of course, as I've frequently had reason to mention, characins possess a very unique "sense" that is demonstrated in several ways, and this seems to be connected to the Weber's Apparatus that this family of freshwater fish possesses.
A second observation is the effect a significant partial water change has on the fish. I have never known any fish to respond other than with increased activity on the day following the pwc. As mentioned above, spawning frequently occurs, and even if not actual spawning there is a marked increase in the level of all activity such as chasing their companions, more eager food hunting and eating, etc. My earlier post on large water changes was referenced in a related thread, and in that post I had a bit more detail on the effect of larger water changes in reducing pollution which is always present and can only be diluted via a significant water change. It is not a stretch of the imagination to conclude that this in itself has a profound effect on the fish's system and clearly in a completely positive manner. There really is no substitute.
Just a final note on plants. In my 20 years of doing 50%+ changes weekly without fail I have never been able to observe plant issues that could be said to result from this schedule. Of course I can't say that the plants might not be even better without these large water changes, but I am certainly not going to sacrifice the increased health benefits of the fish which I can detect just to possibly prevent some undetectable detrimental effect on the plants. And while it is perfectly true that plant authorities frequently suggest fewer than even monthly partial water changes, they are also careful to point out that this only works in tanks with minimal fish loads. The health of the fish should be first and foremost.
Last edited by Byron; 12-23-2009 at 08:57 PM..