Anubias Rhizome Rotting!

     TANK DETAILS: 38 Gal(US), pH 6.8, KH 3, RUGF, no CO2, 80 W (triton), 
     plain gravel substrate, moderate/densely planted (depending on your 
     definition) and is established 3 years. The set-up is lo tech but 
     growth have been satisfactory for a variety of common plants. 
     Fertilisation has been casual for the most part, but I have been using 
     a trace element mix 3 times a week for the last 12 months.
     PROBLEMS: Anubias are my favourites, I have 4 types A. Barteri, A. 
     Barteri var. nana, A. Congensis, A Coffefolia (sp.?).  The nana is the 
     best established and has been present since day 1, it covers about 60% 
     of the gravel surface. They are having problems. In places the rhizome 
     has rotten away completely in small areas 10-20mm, the plant on either 
     side of the rot seems to be OK. The rot appears on many Anubias plants 
     and all 4 types. Is this an infection, a result of some deficiency or 
     a method of vegetative reproduction..... anybody?
     ASIDE: Measurable water conditions have not changed much for years but 
     most of my duckweed has disappeared also.