Yeah pH values can be tricky to deal with, there are just too many parameters to consider. Everybody who tried to adjust a CO2 device for a freshwater aquarium knows what i'm talking about.
If your tap water is "soft" (below 100ppm) then you have a much better chance to adjust the pH because the water hardeners act like a buffer for acids. In hard water you have to use a lot of acid to lower the pH and it can bounce back up when the water hardeners have neutralized the acids.
On the other hand, if too much acid is present, the hardeners balance the pH at a level for a while but they will eventually degrade and the pH will drop down dramatically. In soft water this effect is much weaker and lower quantities of acids are required to lower the pH. Adjusting the pH also takes longer with hard water because the hardeners release CO2 when they react with the acid. The CO2 lowers the pH until it has been removed from the water. CO2 and oxygen levels are the different ends of the scale, water either contains a lot of oxygen or a lot of CO2. The CO2 can be removed by moving the water with a pump or an air stone.
Once the pH value is stable there are all kinds of other factors to consider for a grower. The 2 big ones are the fertilizers and the grow medium.
But hey, lets make a long story short.... in my opinion, before you get into this whole water chemistry thing and poor acids and stuff into your water you have to make sure that you find a solution that works for you. Your tap water, your grow medium, your range of fertilizers, your method of watering, your tap water temperature..... Little things like adding some lime to the grow medium can have a higher impact than messing around with the water.
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