I appreciate your thinking not to use peat. This would mean to engage in a destruction similar to that which we complain about.
But I should advise you not to use hydrochloric acid, it is much too strong. You cannot handle it with waters of very low a mineral content. Myself, I have made good experiences with phosphoric acid and market solutions harvested from oak tree acid. But, this is not safe at any case. There are brands that enhance the mineral content too much, whereas others are rather good for our needs. You must try. The same product could be differently composed in different countries ad at different times. Mostly, “Sera pH minus” was acceptable and effective in Germany, but in Britain it was not to be recommended.
Surely, the use of natural substances like fresh acid peat wood (“Moorkienholz”), dry leaves of oak, beech or Catappa, or alder cones is most to be recommended. But they have the disadvantage that the darkening of the water maybe too strong (especially in the case of too many alder cones); therefore you should prefer a misture of methods. Then, a few drops (not more!) of a phosphoric acid will help.