$275 is a lot. Not even all my fishes costs that much. I'm a fish enthusiast; not a fish extremist. I'm not that fanatical on spending money like crazy on fishes. For $275, I can buy a almost new 100+ gallons tank or pay my rent to the landlord.
I'm possitive the gravel doesn't change the pH. I have tested it myself before. And if it does, it should be metioned on the bag; I did not buy assorted gravel. The gravel is natural; no colouring/coating added.
The tap water is obviously from the city/town. The test yields 7.0 before and after. The chart scales are either -.5/+.5. Even if the test before and after is different, it would be more or less the same since it the chart can't measure the exact number.
I don't really have to explain this; it's common sense. pH will change in time. Nothing stays fine for ever. What I do not want to know is, why does pH change after a month. It's been a month, I don't expect water pH to be perfect as it was. Also, there's the fact that I want to lower the neutral pH level down to 6.5 for my neons and other slightly acidic fish. I want to know how to make some cheap pH down.