pH question: calling the geek squad...

The Mermaid Queen

TFP Expert
LifeTime Supporter
Mar 28, 2007
Northern KY
I have at the moment a 10 gallon freshwater aquarium. (we have a 90 gallon also, but it has been shut down since we moved).
The little pH test kits always come with a glass tube, and somehow we always manage to break them. So I was guestimating how much pH down to add with each water addition. The fish seem healthy enough, though there are only 3 right now (black tetra, cory, and pleco)

We have a whole house filter that removes the chlorine and stuff, and then a water softener. We also have an RO unit for drinking water.

So I bought a new pH tester (bromothymol blue) (wish I had the TF100!!) and checked our tap water. Way above 7.6, the top of the scale.
Not thinking clearly, I also checked the drinking water. I was not remembering that ALL the water is filtered and softened, not just the drinking water.

Anyway, the pH of the RO water is pretty much neutral, maybe 7.2.

Why the difference?

(Please note I did not put this in the advanced section... need a basic answer!! Chemistry class was a LONG time ago!)

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
San Rafael, CA USA
RO, or reverse osmosis, should remove most of everything in the water -- salt, minerals, other dissolved solids -- though it does leave small amounts so is not as pure as distilled water. I don't know why the pH would change except that RO would remove most of the pH buffering, that is the TA, so the resulting water would be more sensitive to changes in pH from outside influences. Perhaps there is a small differential preference in the RO process for hydrogen ion (which is low pH) to migrate through the membrane compared with the hydroxyl ion (which is high pH).

So I really do not know for certain and all I found from a quick search was that most RO water is typically lower in pH than its input water so apparently what you are seeing is normal.