# Why is my Ph rising?



## Fran (11 Jan 2021)

Hi all, I am setting up a black water tank and using RO water mixed with hard tap water to create a tank of about 100 TDS with pH around neutral or slightly acidic. My RO water is pH 6.8 and I mixed some tap water Ph7 and TDS 400 to get TDS 100. On putting this water in the tank the pH rose to 8. Why is this? There is just wood and capatta leaves in there with a thin layer of ADA La Plata sand which ADA say does not effect water chemistry. Any replies appreciated.


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## dw1305 (11 Jan 2021)

Hi all,


Fran said:


> On putting this water in the tank the pH rose to 8. Why is this?


It is to do with the <"carbonate / CO2 / pH equilibrium">. The pH value is dependent upon the level of atmospheric CO2, and at 400 ppm CO2, the pH value will always be near pH8, when you have <"any reserve of carbonate buffering">.

You have three options:

Use less tap water (hardness is Ca++ and HCO3-), so that raises both dGH and dKH.  I'd aim for about 60ppm TDS (100 microS) or lower.
<"Ignore pH (that is what I do)">, pH isn't very useful as a measurement in soft water.
Add a stronger acid, this will remove the HCO3- ions (they will convert to CO2). 
cheers Darrel


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## PARAGUAY (11 Jan 2021)

I get the opposite ph from tap 7 water descibed by utility company as soft to very soft. I had to check when tempory looking after some fish to rehome lately ph was just above 6. But fish did ok no issues till rehomed


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## dw1305 (11 Jan 2021)

Hi all,


PARAGUAY said:


> I get the opposite ph from tap 7 water descibed by utility company as soft to very soft.


That is because the Utility Company use <"sodium hydroxide (NaOH) to raise the pH">. NaOH is a strong base, meaning that it all goes into solution as Na+ and OH- ions, there is no buffering, no residue of undissolved bases, and that means that the <"pH can fall very rapidly"> once you have more H+ than OH- ions. 

I've never managed to find a good way to describe pH and buffering questions.

cheers Darrel


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## Andy Pierce (12 Jan 2021)

I'm going to upvote Darrel's suggestion to just ignore pH.  I suggest you get your tank set up how you like it, let the water equilibrate for a day or two, then measure the pH and unless it comes out really crazy, or unless you want something in the tank you know is super sensitive to being at a specific pH, to not mess with it.  CO2 gas will also lower the pH if you use it.


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## dw1305 (12 Jan 2021)

Hi all, 


Andy Pierce said:


> or unless you want something in the tank you know is super sensitive to being at a specific pH, to not mess with it


That is why I stay away from real blackwater specialist fish now. I use rainwater in the tanks and that picks up enough limestone dust to raise the hardness to 3 - 4  dKH / dGH. Even that minimal amount of hardness means fish like <"_Parosphromenus spp">, Dicrossus filamentosus or  Apistogramma elizabethae_ wont/can't breed successfully, although maintenance is fine in water of this hardness.


Andy Pierce said:


> CO2 gas will also lower the pH if you use it.


It does, but it causes the depression of pH in a slightly different way by increasing the amount of TIC (as added CO2). This changes the <"pH equilibrium point"> (we've added a very small amount of carbonic acid (H2CO2)) and donated a proton (H+ ion). When we turn the CO2 off the equilibrium point returns to that dependent on <"atmospheric CO2 levels"> and the pH rises.

I'm not a CO2 user, and I know no-one who has run a blackwater tank with water with some hardness and added CO2, so I'm not sure whether that allows successful blackwater fish reproduction or not.

cheers Darrel


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## jaypeecee (12 Jan 2021)

Andy Pierce said:


> I suggest you get your tank set up how you like it, let the water equilibrate for a day or two, then measure the pH and unless it comes out really crazy, or unless you want something in the tank you know is super sensitive to being at a specific pH, to not mess with it.



Hi @Andy Pierce 

I like to think of pH adjustment as chasing a ghost.

JPC


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