I think your lighting may be too low for HC also. (Did I see that right...13watts on a 12gal? )
You could try Marsilea minuta. It does well in low-light and gets about 1" tall, makes a thick carpet. I'd show you a close-up pic of mine, but it has some algae on it (rather embarrassing) . Here's a link to my journal. I'm not sure, but I think the best pics of it are on pages 10 or 11 (I can't see my own pics from my work computer) . Marsilea is in the foreground, left side (in front of the large chunck of wood) :
I am not trying to hijack your thread but I also have had no luck with HC in 125 gal. at 3.5 watts per gal and I dose EI method using dry fertz. Every time I get it, it is in emmersed form and never seems to acclimate, it always melts for me. I have grown glosso HM and currently trying elatine triandra which is looking good so far. Glosso and HM took off like crazy. Damn HC, I want it so bad and I can't grow it.
So in light of that if you are having trouble with HC I would try glossostigma and keep it trimmed well.