Seems to me then that the suggestion of ammonium nitrate would work,
if your filters can convert the ammonia quickly enough to nitrite,
then to nitrate. Other than that, highly dilute nitric acid.

I use now Mg(NO3)2.(H20)6 and Ca(NO3)2.(H20)4 at 50g/l level.
I try to keep an Ca to Mg ratio to about 5/1.
Unfortunately, Nitric acid shall not be available in powder form :-(
I will check the availability of diluted solutions !

GOOD !!!!
What for this gimnastic ??????
I use too only ro water.
Caco3,Kcl,Mgso4,kno3,nacl and this is all.
Simply and affectively !!!