MJB wrote:

I just switched from a timer-based CO2 system to a CO2 controller. I was
getting a wide fluctuation in pH from 7.6 in the AM before the timer started
to 7.0 at the end of the lights-on cycle. Now I'm in the range of 6.9-7.1.

My tapwater is nearly identical to yours. I leave the CO2 on 24 hours. The pH
varies less than 0.2 from end of light to end of dark periods.

I use RO water and just switched from Kent's RO Right to Seachem's
Equilibrium. I also use Kent's pH Stable.

I also use RO, but am happy with RO Right. pH Stable is 100% baking soda (Sodium
bicarbonate). Most of using RO buffer with Sodium bicarbonate, but use the
cheaper Arm & Hammer brand from the supermarket! True confessions: I did buy one
jar of pH Stable before confirming it was baking soda.

I'd like to keep the tank at
about 6.8. I see that there are buffers available that claim to stabilize
the tank there, but I've not had any experience with them.

Do yourself a favor and keep it that way. Set your KH with baking soda,
measuring carefully for the first few weeks to figure out the right amount to
add with a given weekly water change. Then target 20-25ppm CO2, and set the pH
controller to achieve that.

Or return the pH controller and set your bubble rate to do the same thing, 24hrs
per day. Works for me.

In any case, targeting pH is not the right way to think about it in a plant
tank. You want to set your KH, then target a CO2 concentration, which is in turn
measured by your pH. I set my KH at 3, which requires 1 teaspoon of baking soda
in a 30 gallon weekly water change. Then I adjust the needle valve to get 25 ppm
CO2, which I calculate by measuring pH at 6.6 and reading the chart at
http://www.csd.net/~cgadd/aqua/art_plant_co2chart.htm