Hi all, It is, H3PO4 is "E338". It differs from hydrochloric (HCl), or sulphuric (H2SO4) acid, by being a weak acid.
In the case of H3PO4, you added 3 x H+ ions (acids are "H+ ion donors") and one PO4--- ion (PO4--- is a base and why H3PO4 is a "weak acid").
The problem with all these additions is that you are adding things to the water and Discus come from water with virtually no salts of any description.
You can think of it a bit like making tea, you start with water and you add a tea bag, this gives you "black tea", which you might make more palatable by adding milk (white tea) and sugar (sweet white tea), but each substance you add takes you further from the original water.
If your tap water is very hard I don't think acids will really help long term and I would cut my tap water with 3/4 RO or rain-water. There is nothing to stop you then adding a small amount of phosphoric acid (or hydrochloric etc.) to lower the pH.
cheers Darrel