A bit late to the party! looking at your photos on the link, that is Persicaria odorata, known as rau răm or Vietnamese corriander. It is a herb and it also grows in rice paddies, but it is not ’the‘ rice paddy herb.
Rotting stems, or the rotting of any part of any plant automatically implies poor CO2 uptake. Investigate gas injection rate, injection timing, as well as flow/distribution. Plants do not care whether they are in sand as long as you dose nutrients regularly.I'm having problems growing this in sand. The bottom of the stem keeps rotting away. Do I need to pot it in soil and plant the pot in the sand?