To make the water more acid you need to add hydrogen ions in the form of:
Carbonic acid from injecting CO2, tannic acid from adding driftwood or oak leaves or tea bags, or rainwater which is usually contaminated with sulphur dioxide. It will not take a lot to change it to 7.2 but 5.5 is a bit more of an ask. If I remember correctly Coca Cola is about 4.6 and that contains a lot of carbonic acid which is carbon dioxide in solution.