Rotala Stems Rotting

Two weeks ago I purchased some Rotala sp? (they were reddish). The 
store had a "cutesy" name which I don't remember.  Since then about 
1/3 of the stems have rotted from below the gravel to one to two 
inches above the gravel.  Growth of the plants with no rot has been 
good with about two inches of new growth.  Also, the ones with rot 
have also shown similar new growth.  Both the plants with and 
without rot are sending out many new roots from the leave nodes.

Conditions:  110 gallon with:

	Temp=78-81 dF; pH=6.7; dGH=4; dKH=2; 2.2 watts/gal; NH3-nil; 
	NO2-nil; NO3<5ppm; PO4<0.1ppm; and CO2=10-13 ppm.  		
	Fertilizing with my version of PMDD's.  Gravel is 1-3 mm 
	sand 	with laterite added to the lower 1/3.

Other plants include several types of swords and crypts, Bacopa ?, 
Myriophyillium ?, Barclaya longifolia (blooming), Apon. ulvaces 
(almost blooming), and Lobelia cardinalis.  These are all growing 
very well.  Also, I had to recently remove some Limnophila aquatica 
as it was growing too fast and taking over the tank.

I'm puzzled since all other plants are doing well, and the Rotala 
is growing well, except for the rotting base.

When I planted them, I placed the ends about one inch into the 
gravel (no roots were present.)

Any advice is appreciated.

Doug Skokna, in warm Houston, TX.
Where we are 3-0 since Hakeem returned.
And waiting for the playoffs to start.