The Change of Black Amazon Swords

Hi Every One:

I recently get some black amazon swords from Delaware Aquatic Imports.
The leave of those plant are in an oval shape when I first get them. But 
I notice the new leave have a different shape compared with the old 
ones. Generally speaking the old leaf is round and short and the new 
leaf is more sharp and longer. Actually it look like totaly two 
different plants.  The growing rate of those plants is OK.  Most old 
leave decay and new one pop up. Does that mean they need some trace 
elements.  Here is my tank

Size - 55 gal, 
pH - 6.8, (automatic co2 contrl)
Light - 140 watt ( 10 hours )
Substrate - (DuplaRit G 500 grams, the tank just setup 2 months ago)
Water Change - 10% every week

Please post your comment, thank you very mush !