Inexplicable, repeated pH crashes!

"Jason in Oakland" wrote in message
om...
I have a 12g NanoCube, with live plants, a very well-functioning
biological filter (no ammonia or nitrites at all, except a tiny spike
when I change water because of the broken-down chloramine--that fades
to 0 in a few hours). I have 5 lively Golden Barbs, and used to have 2
otos, but they eventually died from the pH crashes I'm going to
describe.

For about 4-5 months, I had NO problems with the pH in my tank. Then
suddenly, the pH started drifting down, from 7-7.5 to below 5. The
strangest thing was that the same would be the case after a water
change, where nicely-buffered, pH 7 water would go in, and then within
24 hours it would have drifted steadily down. I would have to add pH
upper (alkaline) to move it back to a stable, around-7 zone.

Actually, the weirdest thing is that when I test the pH (using Tetra's
pH testing liquid), the pH registers at its value after about 5
seconds, but when I leave the vial out, the pH indication steadily
drifts upward, as if there were something in the water (dissolved
CO2?) that was leaving it, and making it steadily more basic. it is
not unusual for it to drift from an initial reading of around 7, to an
eventual reading (about 6-8 hrs later) of 9.

Yesterday, I did a big vacuum-cleaning of the gravel, and took out
tons of this wildly-growing plant (I think a water wisteria), did a
50% water change with properly-buffered pH 7 water....and today, I
noticed two of my Barbs had white, frayed tail fins. Checked the
pH....well below 5. Checked ammonia: zero. Nitrite: zero. Nitrate:
surprisingly close to zero.

Test the PH of your tap water after it sits for a few hours.

Test the KH in your tank.

Make certain your PH and Nitrate test kits have not expired.

Add a buffer (carbonate) in the interim if you're worried. Coral, Limestone,
Marble Chips, etc.

WHAT is the source of all this acid? If it's normal nitrification,
then why no nitrates? Why does the pH test steadily drift upward when
the vial is left outside?

I feel like I'm torturing my fish, and not knowing what to do is
driving me nuts.

Please, anyone with advice?

-Jason in Oakland, Calif.