CO₂
Reason is you can control its input to a desired pH level drop repeatably with great consistency. Organic reduction due to decay in the tank (Bogwood and Cattapa) will produce CO₂ thus dropping the pH naturally, acid generation through this process will reduce the KH of the water allowing the pH to drop further. You can add black water extract to drop the pH through acidity but in regards to your question that is chemically altering the water with a permanent additive (only rectified through water changing if you drop pH beyond desired level or adding another chemical to swing the other way).
CO₂ is the only thing you can add consistently and to be non permanent if desired levels go beyond those required, you shut of the gas and the levels will equilibriate with the atmosphere naturally, like it never happened.
Gas cylinders and regulators feels like an alien chemical manipulation compared to a natural process, but high CO₂ concentration waters (in nature) are a consequence of very high persistent bio load degredation (rotting wood and leaf fall, not regarding acid addition in this process and KH reduction as the main pH dropping factor), I imagine this would be in volumes not sustainable to allow room for plants.
Easier to add CO₂ to attain this pH drop without using additives (and be safe for animals).
If you wanted to go for a Ph of 5 from 7.9, it's going to be additive chemicals most of the way.
From 7.9 to 7 using CO₂ is easy and safe for the tank inhabitants, unless your KH is crazy high!