Interesting duck weed experience

Just thought that I would forward on an experience that I had recently
with duck weed. 

I was using duck week as a cover in my 55 gal discus tank to reduce the
the light level in the tank, plus it looked kinda neat. Well the duck
weed was doing great and had covered the entire tank, until I decided
to add a few more plants to the tank. After doing some thinning on my
show tank I had a bunch of Hygrophila Polysperma (~20 stems 12" long)
that I didn't want to pitch... so I put some sand in a plastic pot and 
added the pot and plants to the discus tank. It was less then a week 
later that the duck week started looking sick. Within 2 weeks all the
duck weed has died. The Hygro seems to be doing just fine. 

The only thing that I can figure out is that the since I do heavy water
changes on the discus tank (and don't add any plant fertilizer) that the
hygro sucked what was in the tank out and didn't leave any for the duck
weed. By the way, I also don't have any algae in this tank at all.

This is not a problem, but I did find it curious and thought that I would
pass it along.