WHY is my PH climbing??

[Bunnie1978]

I have a 10g set up intended for a trio of apistos. It is moderately planted with low light slow growing plants. Substrate WAS soil capped with Turface Pro League Grey, but I soon found out that the Turface is too light to use as a cap. I removed as much of it as I could and recapped with Flourite. Much better for capping! However, before the change with the turface and still now I have a creeping PH issue. After many water changes, adding in some fresh mopani wood, and adding sulfuric acid (PH Down) and after a couple days it gets back over 7.6.

Any idea what might be causing it?

 

[dw1305]

Hi all,
Sulphuric acid doesn't have any buffering, it just neutralised the bases present in the water, so there is no reserve of acid to carry on controlling the pH. The fact that the pH has risen since you added the acid strongly suggests that there is a source of alkalinity in the tank, very possibly from calcium carbonate present in the soil. If you make a suspension of the soil in R.O. water you can test this with the pH meter.

It may well be that you need to replace the soil with a more acid soil, or alternatively use silica sand to cap a small amount of the "turface" or "Fluorite". These are both calcined clay media and should retain the CEC of the clay component.

I'm using 10% acid leaf mould and 10% clay (calcined or "Kitty litter") capped with 80% sand, and (if you want lush plant growth) a small amount of the slow release fertiliser "Osmocote" in the substrate layer.

cheers Darrel