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Co2 stabilising pH

fish.com1

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20 Dec 2008
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North Yorkshire
Hi,

I've been having a read of a lot of the topics in this part of the forum and have found that having stable co2 say at 30ppm may lower the water's pH by 1 full unit. So say the tapwater's pH is 7.6, after being added to the tank the co2 will reduce it to around 6.6. My question is when doing water changes say a 50% water change is done then the 50% not taken out is still in the tank with the pH of 6.6. Then you add 50% tapwater which has a pH of 7.6. So for a short period of time, until the co2 has lowered the pH back to 6.6, would the pH of the water be 7.1(halfway beteween 6.6 and 7.6)?
Wouldn't this change of half a pH unit stress or even kill the fish?

I highly doubt that the above is right, and i have a feeling that the KH of the water has something to do with it. Can someone explain to me how the pH remains stable when doing water changes?

Thanks.
 

Ed Seeley

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The pH doesn't remain stable. The pH fluctuates throughout the course of the day. On my tanks the CO2 switches off overnight and then comes on an hour before the lights. This causes the pH to drop about 1. Then the lights come on and the plants begin to photosynthesize and use some CO2 which will change the pH slightly again. Then, an hour ebfore the lights go off, the CO2 stops and the pH starts to rise again.

And throughout all this the fish are fine and spawning. The KH of the tank's water is 0dKH by the way - no crashes of pH or wild deadly swings.
 

fish.com1

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North Yorkshire
Ok thanks ed. So the fish should be fine with the fluctating gas levels. I am hopefully going to be using a solenoid on my tank, so when it goes off at night will the pH go back up to around 7.6, and then in the day when it's on it will lower it to around 6.6?

When doing water changes would it to be best to turn the co2 off a few hours before the water change to allow the pH to increase so it matches the tapwater pH? If you didn't turn it off and you did one midday when the co2 has been on for 4-5 hours making the ph drop and then you did a water change with tapwater with a higher pH would this shock from the pH rising so quickly kill the fish. I understand that the fish don't mind it changing like ed say's over the course of a day, but would the sudeen change of the pH from the water change stress the fish? Or don't water changes with difernt pH's affect the fish?

My tapwater is

pH - 7.6
KH - 179PPM or 10DKH
GH- 215+ppm or 12+dkh

Knowing all these parameters what is the pH level likely to fall to in the day with 30ppm co2?
Because my KH is higher would that mean the pH stays more stable?

Thanks again.
 

scriptors

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16 Mar 2009
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Italy
With your KH, you can reduce PH from 7,6 to 7 (at PH7 you have 32mg/l of CO2)

If you want to reduce PH you need to use Reverse Osmosis water (for reduce KH ... and GH also)

For me it's not necessary to stop CO2 at night, good KH level (from 4 to 6) permit to not have any significant modification of PH from day to night (I have just 0.1 - 0.3 PH change from night and day)

When you make 50% of water change, in your tank, you reduce 50% of CO2 concentration and have about 0,3 PH modification ;)

ps. frequently PH modification of about 1, or more, don't kill fish but stress their protective mucose and permit easily pathogen disease like ictyo or similar
 

Ed Seeley

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Rapid changes in pH don't stress fish at all. Rapid changes in TDS can as they effect the osmotic balance of the fish. In practice though there is no danger adding slightly different water to a fish tank - in fact it can act as a spawning stimulus for many species. I have even found this with Tangyikan Cichlids that come from an incredibly stable natural environment.

James has written an excellent post about the chemistry behind pH and KH and adding CO2 which is well worth a read.

Ignore test kits and tables that say you can calculate CO2 levels from the pH and KH of your water - they are inaccurate because they assume that the only thing that effects the pH are acids from CO2 and there are a whole load of acids in a tanks effecting the pH. Use a drop checker to check your CO2 is at 30ppm with a 4dKH solution. This means you only have 1 factor effecting the colour of the drop checker - the level of CO2.
 

fish.com1

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Thanks guys.
So i should be ok to run the co2 in the day turn it off at night, do 50% water changes weekly, with my water parameters, and even the more sensitive species of fish should be ok?
 

ceg4048

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UKAPS Team
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11 Jul 2007
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Yes, as Ed says, there are no negative effects whatsoever in fluctuating pH. Cycling the gas on and off is a standard practice. Even the most sensitive of species have no difficulty with this procedure. Of course what fish have trouble with is if the CO2 levels exceed their comfort zone.

Cheers,
 

Ed Seeley

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I keep wild caught and captive bred dwarf cichlids and various killifish under similar conditions and they do great. I do use RO for them as they prefer soft water, but that will depend on your chosen species.
 

Ed Seeley

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They'll be fine. Stable CO2 while the lights are on will help prevent algae problems and will ensure your plants do well. Going above 30ppm is dangerous for the fish. Personally I turn CO2 off at night to give the fish a rest and prevent CO2 building up at night; others run CO2 24/7.
 
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