I now have had a couple of anubia to rot. The latest one was a large bartari in which the rhizome was around 8 or 9 inches long. The It had long roots and was planted in the substrate such that the rhizome was at least 1.5 inches above the substrate. In this case the middle of the rhizome rotted - the rot area was around 1.5 inches long and i cut it out giving me two smaller plants but this is the third time in recent memory (this past year) an anubia has rotted and i'm not sure what is causing it to happen. I know that if the rhizome is in the substrate that can trigger rot but in all three cases the rhizome was well above the substrate - in one of the cases it was 'planted' in a piece of wood with the rhizome growing along the wood.