Why does pH go from acidic to alkaline ?

[dao]

Hi there,

 

So I have two inert substrate tanks which I manage to keep and breed taiwans in. My problem is that the pH was always around 7.0 - I could obviously lower it with filter media, but the media was burning out real quick. So in the end I stopped adjusting the pH as it seemed a lost battle.

Since I'm thinking of a new tank, but want to get lower pH this time I decided to do a test. Obviously my though was that the substrate might have not been inert after all.

 

So I did the following: Prepared my RO water, as usual with all the stuff I add for a shrimp tank (azoo black water, azoo mineral plus, bt9, old sea mud, salty shrimp), and a second batch just with salty shrimp to remineralize the water. I then used azoo ph down to get the ph to around 6.0, TDS was around 110.

the water was in a cup with 1) driftwood, 2,3)substrate that I have in the tanks , 4) water only.

 

To cut this short both test and in all cups the pH has slowly risen within 24 hours to around 7.0 (driftwood cup was the only one that showed slightly lower pH than the rest).

 

Why is that ? I was under the impression that if there are no factors that rise the pH it should stay were it is ?

 

 

[EricM]

I dont see anything that adds KH, and without any KH your PH can/will fluctuate.  

[Kat]

That's exactly it.  Ph will rise depending on the kh after you add Salty Shrimp to mineralize.  What's in azoo mineral plus?

 

Check your kh after adding Salty Shrimp.  If it shows a kh, your pH will keep rising again, despite if you add pH down.  The kh stabilizes your pH.  If you register a kh of 2, your pH will probably plateau at around 7 or just over. 

 

Bear in mind, the nitrogen cycle consumes kH and so do some plants,  so tank water will react differently than the cup test.

 

I still wouldn't go lower than a kh of 2.  Your water is not stable at all without it and the nitrogen cycle pretty much will halt in a pH lesser than 6.0, so it's not easy to maintain a low pH (unless co2 is used to lower it).

[dao]

Thanks for your answers - the kH of the tanks, and the test cups is 0. I understand it can cause pH fluctuations, but I have these tanks for over a year with 0 kH and the pH is stable so I see no reason to change that. Thats pretty much the same with the test cups - another 12+ hours have passed, and the pH has settled at 6.8-7.0 What puzzles me is, why the pH goes back to that ~7.0 range ?

I was under the impression that if I add acids into the water, and there is nothing to buffer the pH it should stay down?