I love hornwort for its ammonia-sink properties, but I've found it to be a bit touchy sometimes. It doesn't like soft water at all. The only way I've ever flat out killed the stuff was putting it in a tank with a dGH of below 4. This stuff like cichlid hard water. As far as lights and ferts, it doesn't seem to be picky about. I had a sprig sprout in a bucket of water I left in a dark hallway. I did notice an increase in growth when I started using fert, but I expect this with any plant.
I would actually leave the stuff floating. It makes a great shade plant.