I've never liked hornwart very much. I wouldn't bother trying to grow it in any substrate, just let it float. It is easy to grow, but I've had it grow too well for its own good. I had one tank (75 CA cichlid community) where it was the only plant. It started out with a floating bunch a little bigger than a softball, then it would go through this cycle of explosive growth (great for keeping down nitrates) to the point where it covered the whole surface at which point the tank didn't produce enough nutrients for it and would have a massive die off, leaving a little bunch left that would then have tons of nutrients all to itself and grow like crazy again (and so one for months). And when some of it dies, it really does make a huge mess