When we lived in a limestone hard water area (sorry, no parameters) java ferns grew like crazy. I gave a grocery bagful to anyone that expressed any interest. Now I live in a soft water area. We get some mineral deposits but KH & GH are pretty low. Java ferns grow but much more slowly. Java "windelov" is another easy to grow robust kind.
Some stem plants are not very fussy, rotala "indica" might work. Little reddish/pink leaves near the light but often just green. Hygrophilla (hygro) species will probably grow too, but they have a new name to get around US ban. I can't think of it right now. Of course you would not let it get out into the wild nor would it survive your winters. I think it's h. polysperma that's illegal, but not it's varieties like "sunset" etc. Just be careful. H. angustifolis (?) is often available but has never grown well for me.
Vallisneria, the so called "jungle val" (not "giant val) in my club, might grow well too. It is likely at least 2 or more species that grow 30+ inches x 1/4inch wide. (1 is spiralis but not the little "Italian" spiral shaped 1, really v. asiatica, although that might grow for you too). Vals are interesting in that they can use atmospheric CO2 from the water surface. There's an old saying if you can't grow vals try sagittarius, & vs versus.
As others have said, some crypts may grow well, a few actually prefer harder water (usteriana is 1 I think, it gets big!). All of the c. wendtii colors are pretty sturdy, you might try 1 or 2 of those (red, green, bronze, mi oya, etc.) Crypts like root tab ferts.
I won't recommend mosses of any kind. I've had a moss hate on for many years now, they go everywhere & are hard to get rid of. They may grow well in any water...too well, lol;. Along the glass, into filter intakes, on substrate, decor, other plants, just a PITA in almost all places. Beware! Once you have it, you'll have for a loonngg time.
