Rotting Roots?

[dorecs]

Hi,
I've got a two week old plant in the tank which won't stay at the bottom, I weighted it down and waited 2 weeks, but the plant still bobs up to the top. I took the plant out (because it was floating around on the surface) and the roots seem a bit brown to me, i'm not sure whether this is normal. It looks as if the brown color is spreading upwards. 
Should I cut the brown off?
 
Roots:
https/www.flickr.com/photos/86426440@N06/13945981411/
https/www.flickr.com/photos/86426440@N06/13945981772/
 
Rest of plant:
https/www.flickr.com/photos/86426440@N06/13969157063/
 
I'm not sure what species this is so maybe this is completely normal, if so, any tips on keeping it down would be great.
Thanks
 
Tank Specs:  6 neon tetra and 4 Norman's Lampeye killi fish, in a 7 gallon tank with two medium sized plants.

 

[fluttermoth]

It's water wisteria/Indian fern. There are some varieties of this plant that do prefer to float; the roots look fine to me. How did you weigh it down? I find a few pebbles around the base normally works.