Echinodorus species

I have three types of Echinodorus (or possibly just one) and was wondering
how to classify them. One large one has long leaves of about 20" which I
have had for some time and think is E. bleheri. I recently acquired a new
one which is somewhat smaller leaves of a much brigher green color. The
stems appear to be much more fully formed and longer so I think it might
be E. amazonicus.  I have two smaller ones which look like miniature E.
bleheri which I suppose are just not fully mature. The leave shape and color
are very similar although much paler due to the poor environment they had
to suffer at the aquarium store. Does E. bleheri have variable leaf color
and shape depending upon it's environment? Is there a way to tell if what
I have is really amazonicus?
 - Steve