Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
Hornwort tends to fall apart any time there's a major change in tank parameters. Often, just moving it from a tank at the LFS to your own tank is enough to trigger this behavior. The addition of CO2 may do it as well.
I've heard that this is its way of rapidly spreading when introduced to a new location. The fragments travel around, then resume growth; resulting in many more plants in a short amount of time. In nature that's a clever strategy, in our tanks it's only a nuisance.
If any portions look like they're starting to rot, decompose, or turn to mush, throw them out. Keep the rest. Larger fragments that break away can be replanted, but it's easier to just let them float at the surface for a while until growth resumes and they're big enough to handle. You should start to see vigorous new growth within a week or so, meeting your expectations for addition of CO2.
Shinycard, it's my experience that Hornwort is one of the few plants that doesn't tolerate Excel well. It can adapt somewhat over time, but rarely shows full normal vigor.