Several points to be made here, given the subsequent many posts with suggestions good or bad.
Taking the las post first, conditioner will not have any impact on GH, KH or pH. You may already know this, but as you mention conditioner, keep this in mind. The pH is 8.2 and that is where it will remain.
Which brings me to the use of peat, leaves, wood, and other organics. With the KH this hi8gh (13 dKH) you will never be able to put sufficient organic items in the tank to bring the pH down much if at all. I already made the point that the KH buffers the pH to prevent fluctuation. The use of chemicals will lower the pH when they are used because of their chemistry properties, but within 24 (or less) hours the strength of the KH will bring the pH back to where it was initially. This is why these substances do not work, even notwithstanding the harmful effect they can have on fish. Add organic matter, no problem, but it is not going to alter the pH permanently.
Vinegar cannot be used, unless the fish were able to live in such water, which of course they cannot. The vinegar like any acid will lower the pH instantly, but it won't remain there without continually adding more acid at each water change.
And keep in mind that every water change performed with the tap water will further strengthen the KH/pH. If you do dilute the existing KH (and lower the pH though by how much it is impossible to say unt5il you actually do it in that biological system) in the initial tank water, the water used at every water change will have to be the same parameters or the tank water will not be stable. With a relatively small tank, going this route is less onerous than it would be for a 90g or whatever, but it still means having RO (or rainwater, someone mentioned, this is good if otherwise safe and you have enough ongoing) on hand for water changes. And working out how much RO has to be mixed with the tap (it is proportional) to achieve "x" KH and pH.
You may find that unless you use exclusive RO water, and no tap water at all, you will not be able3 to get the pH as low as you want it. There is, to my knowledge, no formula for this as it depends upon the chemistry of the source water and the tank water, and the biology of the aquarium.
Taking the las post first, conditioner will not have any impact on GH, KH or pH. You may already know this, but as you mention conditioner, keep this in mind. The pH is 8.2 and that is where it will remain.
Which brings me to the use of peat, leaves, wood, and other organics. With the KH this hi8gh (13 dKH) you will never be able to put sufficient organic items in the tank to bring the pH down much if at all. I already made the point that the KH buffers the pH to prevent fluctuation. The use of chemicals will lower the pH when they are used because of their chemistry properties, but within 24 (or less) hours the strength of the KH will bring the pH back to where it was initially. This is why these substances do not work, even notwithstanding the harmful effect they can have on fish. Add organic matter, no problem, but it is not going to alter the pH permanently.
Vinegar cannot be used, unless the fish were able to live in such water, which of course they cannot. The vinegar like any acid will lower the pH instantly, but it won't remain there without continually adding more acid at each water change.
And keep in mind that every water change performed with the tap water will further strengthen the KH/pH. If you do dilute the existing KH (and lower the pH though by how much it is impossible to say unt5il you actually do it in that biological system) in the initial tank water, the water used at every water change will have to be the same parameters or the tank water will not be stable. With a relatively small tank, going this route is less onerous than it would be for a 90g or whatever, but it still means having RO (or rainwater, someone mentioned, this is good if otherwise safe and you have enough ongoing) on hand for water changes. And working out how much RO has to be mixed with the tap (it is proportional) to achieve "x" KH and pH.
You may find that unless you use exclusive RO water, and no tap water at all, you will not be able3 to get the pH as low as you want it. There is, to my knowledge, no formula for this as it depends upon the chemistry of the source water and the tank water, and the biology of the aquarium.