The pH is stabilised by the KH (alkalinity). Make sure the alkalinity doesn't drop below 60ppm and you should experience no major swings in pH. I run my Blackwater tank at this level and my pH moves less than .2 per day. Alkalinity is maintained by weekly 20% water changes.
There are a number if ways of adjusting the pH of the water for changes (avoid pH buffers if you can)...
I use RO and tap water in a 1:1 mix which gives me the correct GH and KH values and the pH follows as a matter of course (I run just on the acidic side and see a movement of about 0.03 pH and 0.1'C during a water change)
Filter the water through a peat filter.
Create a Blackwater extract by boiling Catappa bark or peat moss and add it to the water until the correct pH is obtained. Peat moss is excellent for this and the extract obtained is very concentrated.
There are a number if ways of adjusting the pH of the water for changes (avoid pH buffers if you can)...
I use RO and tap water in a 1:1 mix which gives me the correct GH and KH values and the pH follows as a matter of course (I run just on the acidic side and see a movement of about 0.03 pH and 0.1'C during a water change)
Filter the water through a peat filter.
Create a Blackwater extract by boiling Catappa bark or peat moss and add it to the water until the correct pH is obtained. Peat moss is excellent for this and the extract obtained is very concentrated.