Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Sometimes the water company will add something to the water to keep the pH up. Acidic water is bad for the pipes.
During the fishless cycle it is best if you can add the minerals the bacteria need, including the carbonates.
Go ahead and add CO2, if you have the plants in there, too. I would set a fairly low bubble rate at first, and work on adjusting it gradually. Don't bother getting into really high levels of CO2 during cycling. Most fish that you get will not be acclimated to high levels of CO2, and you would just have to reduce the CO2 when you get the fish.
Do not add anything like acid buffer. The pH is not so much the problem, as the low mineral levels.
After the tank cycles you can alter the water to suit whatever fish you are keeping. This means getting the minerals right. Not the pH.
Are you on East Bay MUD? The water is too soft for the bacteria to thrive. Here is how I handled this:
Test the KH, and add potassium bicarbonate or sodium bicarbonate (baking soda) to raise the KH to at least 3 German degrees of hardness, and higher is just fine. The bacteria get their carbon from the carbonates.
Test the GH and add Seachem Equilibrium or Barr's GH booster until the GH is at least 3 German degrees of hardness, and higher is just fine. You can also add some plant fertilizer such as KH2PO4, and trace minerals. The bacteria need similar elements as plants. Don't bother adding any KNO3, though. The plants and the bacteria will use the ammonia, and monitoring the NO3 is one of the things you will be doing to keep track of the fishless cycle.
You could use dolomite, coral sand or similar materials (limestone, calcium carbonate etc.) to do the same thing. These materials act more slowly, though. I use the materials above to prepare the water, and keep coral sand or other things in the filter to act when the initial dose of minerals starts getting used up.
Don't worry about what the pH is doing.
Monitor the GH and KH, and keep it up there as long as you are growing the bacteria.