I have had this happen twice now. I have Hornwort that is living and slowly growing and then the temperature in the tank goes up a few degrees. In my case the increase was from 74 to 78 both times and happened in the course of less than a day due to ambient temperature increase. Within a day, the hornwort just melts away. It's even like it just dissolves and disappears. Very little leaf litter is present and what there is is clear in color and also disappears quickly. The other plants, (Anubias, Java Fern, Bacopa, and Temple Compacta), are unaffected. Hornwort seems to be a very sensitive plant.