I'll do my best to explain this. Water has a chemistry which is due to what it has come into contact with after it fell as rain/snow. Water is the most powerful solvent on earth, it has the ability to assimilate what it comes into contact with, such as rock and organics. The GH is how we measure the hardness/softness of water. It is primarily calcium and magnesium. The KH is carbonate hardness. I never worry much about this, once I know what it is. It has a capacity to keep the natural pH os the water stable. So these three factors, GH, KH and pH are stable in the source water. The GH and KH affgect the pH, so if you do want to change the pH you must know the GH and KH,
For example, if you have say moderately hard water, with a GH of something like 10 dH, and a KH also in that range, they will hold the pH where it is. If the aquarist adds this or that to change the pH, it usually gets nullified by the natural buffering in the water. So you change the GH and sometimes the KH, and the pH will re-adjust.
You have a pH of 6 I think you said. I would be fairly certain your GH and KH are low, creating soft water. You can adjust the pH and GH/KH but these impact fish. Water changes become more involved because you cannot change too much water that has different parameters, though the extent may not be much, depending upon the parameters as they are originally.
What fish do you intend keeping? If they are soft or very soft water--which happens to be the vast majority (all South American and SE Asian species, mostly)--you do not want to be raising the GH/KH/pH if it is suited to these fish. They will have difficulties in different parameter water from that for which nature made them.
So, what fish species? What is the GH and pH of the tap water? KH (Alkalinity) if you know it? These may be posted on the website of your water authority.