PH swing at large water change? - The Planted Tank Forum
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post #1 of 9 (permalink) Old 02-22-2005, 05:16 PM Thread Starter
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PH swing at large water change?


I'm not sure about this issue, how much PH change if I was to do a 50% water change with my tap water PH of 8 and my tank water PH is 7.2? How do the fish handle this anyone know?

I have a 75G tank.

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post #2 of 9 (permalink) Old 02-22-2005, 05:53 PM
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I just asked this not to long ago. Check out this thread:
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post #3 of 9 (permalink) Old 02-22-2005, 06:08 PM Thread Starter
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I've read through the thread and couldn't come to a conclusion. Look like it's 50/50 chance. grungefreek seem to have problem while shalu doesn't. I guess it's better to start with a small % water change, say 20%, and work my way up at 10% increment every week until 50% to see how the fish handle. Look like everyone has different experience with this PH swing issue.

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post #4 of 9 (permalink) Old 02-22-2005, 06:31 PM
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I think the pH swing is going to depend on the KH of your tap water. The pH of tap out of teh pipe is different from the pH after it aerates out or sits overnight. My tap comes in at pH 9 with a KH <1. Since the KH is so low this is not the equilibrium pH. Overnight it goes to 7 as the gases come out and in of the water.
My tank pH is set at 6.8. When i add 50% new fresh tap it goes to 6.9-7.0.

So lets say if your tank kH is low but your tap has high KH, the amount of new water will add more carbonate hardness and determine your degree of pH swing.

The CO2 in my water and the KH from the old water (4) readily buffers the new water and minimizes the swing. I add baking soda to bump the KH up after the pH stabilizes.

You can minimize pH swings by running a powerhead in a tub of your new water to aerate it for a couple hours. That should flush out any gases.


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post #5 of 9 (permalink) Old 02-22-2005, 07:33 PM
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Khoile - I kind of look at my thread this way - WfxXx, Malkore, & shalu all reported no issues. I have read a lot of posts by these folks and I respect their opinions. grungefreek was the only one who reported issues and he was using DIY co2. I have a controller/pressurized system that I am setting up so I hope it is not an issue. I am pretty confident that it will not matter. My tank is still not up and running (don't ask ) so I can not yet speak from personal experience.
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post #6 of 9 (permalink) Old 02-22-2005, 08:00 PM
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Yep i was having big dramas with pH swings at water changes. Since i have started lowering my change water pH to the same as my tanks, i have stopped losing fish. Dont know why i had issues and others didnt, but pH seemed to be my problem.

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post #7 of 9 (permalink) Old 02-24-2005, 04:00 PM
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FWIW, my parameters: KH=7, GH=10-11, tap pH~7.8, tank ph~6.9. I've been doing weekly (in theory ) 50% water changes for nearly a year now and have noticed any related loss.

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post #8 of 9 (permalink) Old 02-24-2005, 10:33 PM
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Having done 50%+ water changes for some 30 years, (non planted tanks as well), I've had no issues in various tap waters all over the USA.

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post #9 of 9 (permalink) Old 02-24-2005, 10:59 PM
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I think the main reason of ph being a problem regarding water changes may relate to osmotic pressure changes on the fish.
Water hardness is the real issue I believe. Ph would coinside with hardness IF we were not adding co2, Sooo if the difference in ph is because of the co2 I dont think its as big as an issue. Anyone please correct me if Im wrong
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