This is a biorb. Even if you can't "plant" any plants, you still have options...
Floating plants- water lettuce, frogbit
Rhizome plants- Java fern, Anubias (tied to rocks, wood, etc)
Mosses- Java, christmas, flame (again, tied to things)
For ammonia, you could have just left one of the dead fish in the tank... Or just buy a male guppy to cycle the tank. After a week, test the water. If ammonia and nitrites are 0 and you have some nitrates, then you can add another fish (after a 25% water change) If you don't have nitrates or if your test shows niTRITE, then it's not cycled yet, just leave the guppy in by himself for another week.
Be careful not to overfeed when you're doing a fish-in cycle.