I would not use Hydrochloric Acid.
My neighbor is a chemist in the Nuclear business… He recommends Phosphoric Acid as also mentioned by Peter from his early days of adjusting pH.(it has no affect on the TDS) According to my neighbor, Phosphoric is a good organinc acid that will not harm fish, plants etc. as long as used with great caution ie: just a drop or 2. It can crash your pH if not not used by the drops (small drops) He has been very helpful in explaining ph and possible ways to adjust it safely. He brought some to me to try.. it works well.
As to using tannins, my technique of percolating peat moss is working wondefully. I keep the peatmoss in a large container soaked in RO water. After about 3 weeks “perc” it thru an old electric coffee maker thru the filter… mix it into RO water to get he darkness you want. The neighbor is really into, the Paros Project but unable to keep any fish due to time constraints so is helping me in anyway he can outside of the actual fish keeping.