Get some tank water and put it on a small dish bowl and let it sit for 48 hours.
Then get the pH reading of that water that has been sitting and degassing CO2. This will let you know what your tank water's pH when the CO2 levels are at minimum -- a baseline of sorts.
Depending on your lighting, a good start would be to set your pH controller to a full pH point below the "baseline" you recorded. So let's say your de-gassed tank water is still reading 6.3, then set your pH controller to 5.3 (if it gets that low), but WATCH your tank inhabitants the entire time.
I'm thinking your peat moss is bringing your pH down a LOT on its own.
A while back, my tank water was at 7.6 when de-gassed. I had my pH controller set to 6.0, as I was derping around with very high lighting during that time.
Now that my tap water has gone down in pH (probably less treatments this time of the year?), my tank water is at 7.0. Since my lighting is also reduced now, I have my pH controller set to 5.8. No problems with my CRS, RCS, amanos, dwarf pencils, and dwarf rainbows.