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I have an aquarium with some rocks and substrate mixture, and about 15 small fishes.
The pH level of the water is always high (7.6 - 8), away from the recommended value for the fishes I have. I want to fix the pH level permanently; I don't want to just add chemical components for a temporary effect.
How can I permanently reduce the pH level?
You should consider adding some natural pH reducing element. Adding drift woods, indian almond or oak leaves will slowly leach tannins, which in turn will reduce the pH. If you have a planted tank, you can start injecting CO2 which will also lower the pH. Another option is to filter the water through peat moss or commercially available peat granules.
Double-check all your rocks: many, like limestone, have enough calcium to keep your pH high. The usual "vinegar test" is to see whether the rock fizzes when it comes in contact with a drop of acid -- vinegar or ideally stronger. If so, it's definitely affecting your pH.
That said, don't worry too much if you can't get it super low. Don't assume every fish you see at your LFS will adapt to your water, but many fish will as long as you're on top of regular maintenance. Some of the more common commercially-available fish are domestically bred in water conditions that don't really resemble what they "should be" on paper.
So bring it as close as you can, but I'd avoid any drastic measures, especially anything you'd have keep adding and adding. If your tap water and tank conditions have a steady-state pH that's a bit high, it's easier for your fish to adapt to that once than it is for them to have the pH repeatedly go up and down.
What is you tapwater pH, and carbonate hardness (KH)? Without knowing this it can be hard to give advice. If your tapwater pH is high every water change will pull the tank value upwards.
Adding to the answer on naturally pH-reducing elements, I can also add Alder cones, which can lower the pH and will also stain the water. I recently did a test where 8 alder cones in 0,3l of pH 8 water (KH ~5) dropped to about 6 in one day, and to 5 or below (my test didn’t go further) in 2.5 days. Cones, leaves (e.g. almond/catappa, oak, beech) and peat can all both be used either inside the aquarium, inside the filter, as water treatment when doing a water change or to create a pH-reducing concentrate. "Fixing" the pH is probably not possible though, especially considering the water changes. Every solution will have to be monitored and regulated to some extent.
