Likely the salt, though it could depend on what kind of plants you've tried and what your lighting is. How come you're adding salt?
These plants should be fine in low light (but you probably want to get the salt out): (Floating) Dwarf water lettuce, duckweed, hornwort, (attached to driftwood or rock) anubias nana, java fern, bolbitis, java moss, (planted) Elodea/anacharis (this can also be floated), vallisneria, cryptocorynes, swords (some of these can get quite big like amazons but in a monster tank that isn't much of an issue), dwarf sagittaria, aponogetons (again some can get big like ulvaceus and you want to research these since some can be picky), watersprite, moss balls are another easy one and it just sits on top of the substrate, you want to turn it over occasionally so the bottom doesn't die.
Hope that helps a little