Good move on buying your own test kit. Have you tried testing your tap water after it has degassed overnight? It will often change after it comes out of the tap. Imho
pH is one of the things that are worried about too much. As long as the tap water isn't swinging wildly, almost any freshwater aquarium fish that isn't wild caught should be perfectly fine to keep in your aquarium since they are all very adaptable to the differences in water.
Here is what I would do to get a base line reading on your pH
1. Test water straight from tap
2. Test water that sits overnight
3. Test water in the morning as soon as your light is turned on
4. Test water halfway through your light cycle
5. Test water before turning off the light
Doing all of this will give you a good idea of your pH averages. If you are noticing large fluctuations in your tank pH during day then it would be prudent to look at solutions to add some buffering capacity. But until then I would stop fiddling with your pH until you have something to compare it to.