pH is one of the most commonly misunderstood and abused topics that show up on these internet forums. First things first, if you had a pH of 8.0 in the morning and a pH of 8.3 in the late afternoon, you are fine. pH is not a fixed number and will fluctuate. Your lowest reading will be in the morning and highest late in the day. Using a refugium with a light on the sump, opposite cycle with the display, can help this, but is simply not needed.
When you hear not to use pH buffers do not confuse this with alkalinity buffers. They can in fact be one in the same, and it is HOW you go about using them that makes all the difference in the world. You need to be testing for calcium and alkalinity, and using an alkalinity buffer and calcium supplement. It has been several months since I discussed this on a thread, so i will elaborate here. However, for anyone who is interested, there is an article posted which goes into more detail:
My personal routine is to test for calcium and alkalinity every week. I record the test results so that I can establish trends. I use Kent Marine liquid Calcium chloride and Kent Marine Super Buffer DKH. Kent Marine also offers a product to adjust pH, but this is not what I am trying to accomplish. I do not want to adjust pH, I want pH to remain at the correct levels based on establishing correct alkalinity and calcium levels. Sudden pH fluctuations are very stressful and often deadly to livestock, which is enough reason for me to not mess with pH adjustments.
When testing I am looking for a target range of Calcium at 440 to 460ppm and alkalinity at 10 to 12 DKH. There are 5 possible outcomes:
1) Alkalinity and Calcium are both low. Action: add Calcium Chloride and buffer.
2) Alkalinity and Calcium are both high. Action: do nothing.
3) Alkalinity and Calcium are both within the target range. Action: wait a couple of days and test again, attempting to determine when and how much Calcium chloride and buffer I need to add to maintain this stability.
4) Calcium is low and alkalinity is high. Action: add Calcium Chloride.
5) Calcium is high and alkalinity is low. This is rare. Action: DO NOT add a buffer. Water changes are required. Anyone wanting the chemistry behind this should do a google search on "Randy Holmes Farley" and read his stuff. Beware, your brain will freeze from the complicated chemistry involved. In a nutshell, your minor buffers have fallen out of balance with your calcium level. A series of water changes is generally the only way to correct this. Again, this is rare.
Hopefully this helps.