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I had a problem that the pH dropped from 7 to 6 and the KH went from 3 to zero and dropped again even after adding KH buffer ( Kent marine pH stable).
The only thing different was that I used carbon granules (in the external filter) from a different shop.

I removed the carbon and did a water change and everything is OK now.
I now wont use carbon, is it possible for carbon to lower pH ?

Are you certain that they were carbon only? Peat granules will affect pH, but I have not heard of carbon doing much more than reducing DOC and removing trace minerals. If there were unknowns mixed in with carbon, who knows?

Pete,

Carbon is often given an acid wash at the manufacturer to get rid of impurities. I would assume that you just got a bad batch which was not properly rinsed afterward.

Larry Vires

thanks for replies it was definately the carbon as the pH is stable now after taking it out.
I have always used carbon to keep the water clear, as there is bogwood in tank
I wont chance using it again as the fish got stessed out and I lost one angelfish
But they are all OK now