buffers can cause pH swings and a higher consistent pH is better then a pH lowered by buffers which could have a nasty swing possibly killing your fish...
You could get driftwood from the lakes/oceans and boil it for about a week if you dont wanna spend any money. But lfs brought driftwood allows you mroe choice.
With opeat, you just out it into your filter and it lowers your pH. But be careful, low pH's cannot usually maintain a steady pH for long.