Stem Plants

This question will reveal the depths of my ignorance about aquatic plants,
but does anyone have an effective way of keeping non-rooting stem plants,
like hornwort, from getting loose and floating to the top of the aquarium.
I can't seem to get those lead weights to work for me.  I presume that at
least one of the following is true:
a) sometimes I put the weight on too loosely and the plant slips out
b) sometimes I put it on too tight and the plant rots at the bottom, then
comes loose
c) I am trying to do something unnatural for the plant.

Department of History
University of Iowa