So basically I screwed up?... So I shouldn't use buffer... When should I be concern with ph? I'm really worried about hammer heads
Only use "buffer" to raise PH if your Alkalinity is low. That is the ONLY time it is appropriate in a display tank.
PH should be treated as a symptom, not something you can directly fix. This of low PH as a Flu. Sure you get a fever and you can reduce the fever with medicine, but reducing the fever doesn't do anything for eliminating the Flu, it could in fact extend the life of the Flu.
Same with PH, you can mask the real problem by temporarily and artificially raising the PH with chemicals (Borate or Carbonate). That does nothing to fix the problem though. In fact, it can make it impossible to find the real problem.
Low PH can be caused by insufficient Carbonate (please list your Alkalinity level). This isn't usually the real cause of low PH though. It is most often excess CO2 or metabolic acids from decomposing organic matter.
To check if it's CO2 simply get a glass full of tank water, test the PH of the water in the glass, take the glass outside for 10 minutes and retest the PH. If the PH rises then it's simply excess CO2. Solving that problem could be as simple as cleaning your skimmer.
Metabolic acids are more complicated, you need to find the source. Clogged filters with decomposing detritus, rotting leftover food or fish poop, etc.