I assume Red Hornwort is a variant of Ceratophyllum demersum, a floating weed. Many place a weight on a bundle and use it as a submerged plant. A word of warning is that anything stuck into the substrate will die and rot. These guys can generate a lot of plant waste and be a pollution problem.
It grows rapidly an inch (2.5cm) a day is normal.
They are said to produce a substance that inhibits algae, I do not know about that.
I keep all my Hornwort the way you described it, tied up to small rocks and yeah, it grows fast. I only keep the crypts, the swords,the wisteria and the rotalas deep in the substrate . My wife noticed that the red Hornwort is starting to get a little green here and there, probably due to the lack of pressurized co2 but, I will enjoy looking at it while it remains red.
I use plenty of fertz including Iron. Just tested, nitrates are at 20. Should I bump it up to 40?
I cut down on po4 since I got rid of bba about 2 wks ago. I only dose about 1ppm 3 times/week for a total of 3ppm/week. I used to dose po4 about 7 or 8 ppm total/week. I have soft water in NYC and I also dose Ca, Mg, Barr's booster and Equilibrium. Also, 4:1:1 solution of csm+b, DTPA and EDHHA(super Iron 138).
My wife bought 2 large Echinodorus uruguayensis on Saturday and this morning she noticed that 3 or 4 out of about 20 leafs are turning yellow at the tips and in the middle (no pin holes ) while the rest look very healthy. Is this the adjustment period or phosphate and/or nitrate deficiency ? All the other plants are OK.