How do I lower ph in my fish tank?

I have tried different water (spring, distilled, tap, RO, even mixing them). My tap water Ph is high, I found out, so I don't use it now. The water I get from the store has a PH of 6.6-7.2 generally, but 24 hrs later after adding it to the tank, it jumps up to 7.6 or higher (my test kit only measures up to 7.6). I don't know why this keeps happening. The only things I know of to do is just keep changing the water (but, remember, the ph just jumps up 24 hrs later). PH Down only works for a day and then the PH jumps back up. It is a waste of money. Anyway, the stores in my area are not helpful. they can't even tell me what the actual Ph is because they use the same limited test kit that I have. I need a buffer they tell me, but, no one sells it. I have looked online and only end up frustrated-not knowing what to buy or not to buy/what to do or not do. Those CO2 catridges are expensive! I do not want to use peat moss or bog wood-they brought ph down too much and turned the water pee-yellow. Not to mention the smell. I have wondered about using Baking Soda. Some say it raises the Ph, some say it will lower. How can it do both? Also, what is the prescribed amount per gal? Finally, what is this about using vinegar to lower Ph? Does the kind of vinegar matter? Again, like with the baking soda, how much vinegar per gal of water? Any tips will be welcomed. Thanks