I guess that the description I gave above was a tad vague…what I mean is this:
Step A: A container separate from that the fish would occupy is initially filled with a large quantity of distilled/RO water. I agree that the hardness/ph spike from doing this in the main tank would be a problem, and I thank you for pointing that out.
Step B: In said tank, a filter is installed that will have a replacable container with an acidifying substance (likely sphagum peat moss…leaves in the bag might look a tad ridiculous and would not be as efficient if you think about it). The container, which I will refer to as a ‘battery’, would be replaced about once every two weeks.
Step C: Relatively small amounts of dechlorinated tap water both initially and to replace the distilled as it was used (maybe 1-2 gallons at a time per every 15-20 gallons of the container?) would be added to the container. Since the buffering substances would be so strongly diluted, they would be neutralized trying to contain the acids and the PH and hardness would remain low. If this works as I think it would (I could be wildly off the mark here, though), as long as the battery is replaced consistently and tap water is not added in excessively large quantities, there would be no need for RO or distilled water after the system is initially set up. This could be a huge money saver for acidic water keepers in general – if it works, which is why I made this post.