Can carbon affect pH?


New member
I just bumped up the amount of carbon (GAC) in my tank. There are some thoughts that GAC may play a role in solving tridacnid clams' pinched mantle. This is yet to be verified, but we're testing. The idea is to run as much GAC as feasible. I used to run 3/4 cup in my 38g. Right now, I'm running well over 3 cups, and closer to 4.

Lately, my tank's pH peaks at around 8.15, and dips to 7.60 at night. (Yeah, I know, that's a whole 'nother discussion). Last night, I added the GAC at about 6:00pm, when the pH was about 8.10. By 7:00pm, it was already 8.25, and by lights out (10:00pm), it was 8.40. This morning, pH had only dipped to 7.80.

So I got about a .20 bump, across the light cycle, and a particularly quick spike late at night. Has anyone ever seen this before? I can't think of any reason GAC would affect pH, even as simply an influencing catalyst to something else.

38g, no sump
Ca: ~400ppm
Alk: ~9dKH
Mg: 1350ppm
SG: 1.026
Top off: 100% kalkwasser through reactor
Yes, carbon can impact pH if you use a lot, but the effect won't last long. It is often acid washed at the end of the process, but the extent of that washing may impact the subsequent pH.
I know, but a less extensive acid wash may leave the pH high. :)

This article is referring to freshwater systems, but the point is the same:

"Acid-washed carbons will leach less phosphate than others and are usually more expensive, since they have had much of their ash and phosphate washed out. Ash is important because it is responsible for "pH shock". Some carbons can increase pH to over 10 in a very short time. An acid-washed carbon will barely increase pH to 7 over several days. Carbons that do not alter pH are usually the same carbons that will not leach much phosphate."
sorry to highjack this thread...

is it GOOD or BAD to run Carbon on a Phosban reactor?

I have a 75G tank and I run 24hr phosban, and 1hr my UV 9w, I don't have any problems, and the water is cristal clear, but should I run it?