Adjusting high PH


Junior Poster
Aug 1, 2007
Dallas, TX
I have a friend with no internet access who asked if I would post the following question:

He lives in an area where the water from the tap has these two parameters (he hasn't tested for KH yet, but I've asked him to). But for the purpose of this post, let's assume that the KH is high enough that CO2 injection is going to change the PH little or not at all.

PH 8.0
GH 13 ppm

Based on a recent thread I had that Vaughn responded to (among others) the suitable range for a planted tank is between 6.5 and 7.5. Since his PH is around 8.0 what should he do to reduce his PH to a suitable level?

Thank you,


Lifetime Charter Member
Lifetime Member
Jan 24, 2005
Sacramento, CA
It is unusual to have such a low GH and such a high pH. Are you sure that the GH is 13 ppm and not 13 dGH? And, contrary to what so many people post here, a high KH does not keep the pH from dropping as you add CO2. If you manipulate the equation that relates CO2 ppm to KH and pH, you get:
initial pH - final pH = log(final ppm of CO2 divided by the initial ppm of CO2)

So, the change in pH for a given change in ppm of CO2 is not related to the KH at all. KH is important only because some plants need low KH to do well, and some need moderately high KH to do well. But, most plants do well over a big range of KH.

If that were my tank I wouldn't worry about having pH 8.0 tap water.


Plant Guru Team
Lifetime Member
Sep 23, 2007
South Florida

My tap water is 7.8 - 8.0 here in S. FL and I have a planted tank.

I do use auto c02 to reduce ph to 6.7, but have another tank with NO c02 and the plants/fish do fine with it.

Growth is just not as lush as with fertiilization......

I also try and use fish/plants that match my water, rahter than try and keep softwater/low ph fish and plants....

Peat moss is also an option to lower ph and hardness, but will discolor the water with released tannins......

Hope this helps.