My pH level is 6.4 when the lights and CO2 are on, the pH is 7.4 when I test it early morning when the lights are still out. I also have an airstone connected to an airpump running at night when the lights are out for some O2 for the fish.

If I put peat in my canister filter,will this give my water a steady low pH even at night ? Should I use Seachem's acid buffer?

Not overly sure why your worrying, CO2 in the water doesnt mean that theres no O2 in the water, one doesnt replace the other..

infact.. during the day, when the plants are photosynthisising, your O2 levels will be higher than they would be with no CO2, as the plants are producing a shed load of it.

Just make sure you have good surface agitation all the time and you should have no problems, that wont get rid of much CO2 during the day, and itll keep O2 levels nicely for the fish, and allow it to sufficiently de-gas during the night. O2 is highly insoluble, so the airstone isnt like to do as well as good surface movement.

Lights can drive the pH down this much if you use a pH meter.
If it's a test kit, then it's mainly of issue of when you test, all that matters is that you have good CO2 when the lights are on, adding anything else will not help.

We add CO2 to fertilize the plants, not to control pH.

Regards,
Tom Barr