Tropical Fish Keeping - Aquarium fish care and resources

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stephanieleah 12-22-2009 11:19 AM

WC and pressure changes
I'm clipping this from my other thread hoping to get more replies:

Does doing larger water changes impact the fish's osmoregulatory system? My reason for asking:

I used to do 30% water changes but now I do 40-50% because I am now fully stocked. I was thinking that taking out half the water makes a pretty sudden change in the tank--the bottom layer (ie the one with the most weight on it) is now gone and there's only "lighter" pressure in the tank (for my pleco or loaches, for example, that would be robbing them of their pressure zone pretty suddenly). (Kind of like us going on an airplane and the ear pressure deal).

I started doing a back to back 20-30% WC to 1) lessen the pressure differential during the WC and 2)keep my filter going because right around 30% it can't suck up water and makes a weird noise that's hard on the motor. Does this matter? Does the osmoregulatory function work faster than the rate of the pressure loss during the WC?

jaysee 12-22-2009 11:26 AM

The pressure difference between being in 10 inches versus 20 inches of water is miniscule.

Angel079 12-22-2009 11:38 AM

While I also believe the pressure diff is minimal, I also do believe that a w/c should rather be less water and more often then a seldom w/c @ 80%. Its simply more natural, you'll find with some fish's origin that they almost have daily / often rain showers which we can simulate with the w/c and also its much better to remove waste often and a lil then let it sit there and only clean it let's say every 4 weeks and then hardcore.
Also on a side note when doing 80% and then some w/c you're pretty much removing all CO2 necessary for the plants and these up & down swings are also not good for plants to thrive.
Sometimes large w/c are not avoidable (eg medicating tank) but on a day to day bases I sure believe one should aim for w/c more often and then less water ;-)

Edit: Not to mention that having a tank with w/c only every few weeks surly does promote some nice algae growth too :-)

Byron 12-23-2009 07:49 PM

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.


jaysee 12-23-2009 09:23 PM

Excellent post.

As I always tell people, when in doubt do a waterchange.

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