Anacharis Rotting

[JohnDyer]

I have a few stems of Anacharis in a 10 gallon tank. They've been there for about 2 weeks. The main stem planted in the sand always rots, but the branches are bright green and grow very quickly, nearly a centimeter a day. I don't think it's lacking nutrients because of how fast the side stems grow. I have 2.6 wpg, which should be enough. My water is a bit chilly, 65 degrees, but the dwarf sagittaria is doing fine.
 
Is this normal? I feel kind of silly, Anacharis is supposed to be unkillable, but I'm managing to kill it anyways. 

 

[BerryAttack]

try bumping up the temp to 75. the sand will always be colder then the water, even though it is soaking. it doesn't allow the transfer between heat so well then just in free water. if it is growing then you are fine. probably the substrate is to cold for it to grow roots.

 

[SO19Firearms]

the sand will always be colder then the water

Is that true? Possibly marginally, but if it were a factor at all that would imply heater cables have a use - but they don't.

 

[BerryAttack]

 

the sand will always be colder then the water

Is that true? Possibly marginally, but if it were a factor at all that would imply heater cables have a use - but they don't.
 

When I had sand in my 40 gallon, I would keep the temp at 26 for the water then when i would move around the sand to allow trapped gas etc when doing a water change it would be cold. I took a temp of the sand for fun one day and was lower then what the water temp. the stand exposes the glass on the bottom. 
 
but as the plants are not rooting into the sand and the water temp is 65, then there can be the cause of the difference of temp as everything else is growing well. Just a thought anyways.