SOrry to break the bad news to you, it's Just hornwort is not a rooted plant, don't grow roots at all.
If you have good lighting, and use ferts it will do fine weighted down, and will grow like wildfire.
Planting it in gravel will kill that part of the plant, then when you fish start dieing from Amonia Poisoning, you can;t figure out what is going on,
Get some plants wieghts, and remember not to put them on tight, put them as in a way the plant can still move, pinching them with wieghts will cause that part of the plant to die also,
Floating them is pretty easy, and they will grow like mad, I use either a suction cup with a hook-small ones, or driftwood with a suction cup and stick it at the water level and drap them over it also.
If you have Allot of lightning, they should do ok wieghted down, if they start to urn light colored and not looking well with good lighting, float them near the lights for several days...
Hornwort is one of my favorite plants, and I have always had it sence early 90s....both store bought and wild......