1) what test are you using?
2) R/0 or an R/O mix is the only way to 100% control PH levels imo.
3) driftwood, peatmoss, almond leaves ect can drop PH a few ticks. .4 is the largest difference I've ever seen in my own tanks. high 8's these are likely to be useless.
4) what physically is in the tank? certain rocks/decor ect can do some serious buffering.
5) whats your KH reading?
6) If your still stumped, get on the phone w/ your water treatment facility. If on a well get your county on the phone and have a well test done relatively cheap. iir cost about 20$ for us to have it done.
7) Chemicals may work to check out if your deco will impact it.. But I wouldn't reccomend dumping them into a tank w/ fish. Chemicals will quickly dissipate if you will and jack the PH back up usually within' 24 hours. Faster if your water is hard. As stated, steady PH is alot safer then PH fluxes in a tank. Even if just used to check the deco the chems instability, I woudln't trust teh results. maybse use it as a guideline..
Personally, not sure how big this thing is.. But I'de just go to the store and spend a few bucks on some Disstiled water. prolly cost you less overall or about the same as Chemicals woudl to treat your own water. My local LFS sells R/o water as well and it's 50 cents a gallon if you bring your own bucket. not sure how much the grocery stores does, But most have Systems now for the same or similar application. So if it's just for the test this might be your best route.