Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
True, there are several reputable research papers saying that as pH drops below 6.5, nitrobacter and nitrosonomas (your biofilter) shut down then die.
And I do observe what appears to be a reduction in the bacterial colony in HOB filters that I use as CO2 diffusers.
But as you have observed, some people run this low without actual problems in real aquariums. Why this is, I don't think anyone knows for sure. In a planted tank, plants can take up the role of primary biofilter, especially with the added growth from CO2. In discus tanks, something else must be responsible; it could be that bacteria adapt over time, or another species we're not familiar with moves in.
But forget the statement about "pH has to drop a full degree to have acceptable CO2" entirely. pH is a logarithmic, not a linear scale. Every 1 point drop from 7.0 represents a tenfold increase in acidity, which makes this rule incorrect, and predicting your final value difficult.
Your tapwater's KH of 3 is plenty of buffering capacity to avoid a pH crash (a rapid, uncontrolled drop in pH, when the ratio of acidicity/alkalinity gets too far out of balance). Stop adding buffers, and you'll probably end up at 6.2-6.4, rather than 5.8. I've run at 6.2 without problems.
So I say go for it. But I would test ammonia levels and watch the tank after the change, just to make sure.