Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
"Tap water 6.6 to 7.0, sometimes up to 7.6 and neutral substrate... "
Not enough info. What buffers are in the water? Carbonates are the most common. Test the KH (or look it up in your water quality report).
Are you using any pH altering materials? (pH up, neutral regulator, pH down... )
Peat moss, certain substrates (test the pea gravel to be sure it is neutral).
Changes in pH are dictated by what is in the water, and what reactions are going on in the water, for example what the microorganisms are doing.
When you are cycling you are mainly growing nitrifying bacteria.
These bacteria need a small amount of certain minerals. They get their carbon from carbonates.
The KH of the water can go down as the bacteria use it, but such a small amount that unless you are starting with almost none the loss is nothing. Just make sure the KH is at least 3 German degrees of hardness, and your tank will be fine. Higher is not a problem at all.
Other things going on:
If you have added to the tank or filter peat moss, driftwood, organic soil (meaning compost of some sort), leaves or other things like these, these materials release several organic acids to the water, some act almost like ion exchange water softeners, and decomposing matter has a pH lowering effect.
If the KH is already a bit low (3 degrees or less) these other effects can lower the pH.
Test the tank while it is cycling. Keep the GH and KH over 3 German degrees of hardness. The nitrifying bacteria grow best when the pH is on the alkaline side of neutral (anywhere in the 7s), but I do not know if that is strictly a pH thing, or because the bacteria need carbon from carbonates, and if the carbonates are too low the pH is often too low, too.