You won't get that much of a drop by using Pete moss. Even then it will most likely be a slow process. Even if you do achieve your desired ph using Pete moss water changes will become problematic as you will most likely be adding tap water that could drastically alter ph.
How much it drops depends on how much you had and the buffering capacity (kh) of your water. To have a ph that high usually the kh is fairly high to although not always true.
Why do you want to lower ph? Most aquarium fish will adapt to most water supplies with a slow acclimatisation process although I would take extra care with ammonia traces with a ph that high.
If you desperately want to reduce the ph if your water you would be better off cutting your tap water with RODI water as you can keep the ph as stable as possible with each water change. Failing that. Choose fish that match your tap.
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