The plant to me looks like it probably is one of the Taro varieties eg Colocasia http://en.wikipedia.org/wiki/Taro. Taro plants have large elephant shaped ears like that so is also commonly called "elephant ears".

I have planted a black stemmed Taro (the green stem wild growing variety is more common in my bog area of my garden and if I was currently able to post a picture of it, you'd see the plant other then the stem colour looks the same. This kind of plant which can be growth in an aquatic area or in a normal garden, would get too big for aquariums and outside in an water area, it can become invasive.

If it is a Taro, taro plants when disturbed can release toxins harmful to native wildlife according to this article http://www.saveourwaterwaysnow.com.a...ls.asp?ID=1790 . The sap is toxic (Thou I have mine in my bog area outside, due to it being a toxin releasing plant, I wouldnt put it into the pond itself. When I get my garden more set up, I plan to move it out of the bog/frog area as its very likely not to be good for frogs seeing it is harmful to "wildlife" and can be harmful to us too.. if handling it in a garden, pulling it out, gloves should be worn.. thou this plant can be eatten too if cooked right).