Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
That is an extreme drop!
Are you sure it was the EC? What else could do this?
Were you monitoring the KH through the episode?
EC used to have a problem with raising the pH, but that was many years ago, I have not heard of such a problem lately. The manufacturer probably removed whatever buffers they were adding that raised the pH. Fine, as far as it goes, but then there would have been nothing to counteract whatever caused the pH drop in this instance.
Distilled water also has no buffers. If you had initially filled the tank with DI, then a small amount of acidic material (peat moss, IAL, alder cone, oak leaves) could drop the pH. I would not think it would go that far, though. I have had peat moss take the pH down to about 5.0.