What are those Fern like things?

I have had my little pond for three years. During year one I put these
fern like plants in it. I planted them in a little bowl of sand/dirt and
placed them in the shallow part of the pond. After that they took off and
completely covered my pond. I loved it! Well, last winter was really bad
and the ice killed off all the plants. I was just wondering, what are those
fern like plants called? They grow like crazy! Anybody know of any good
web sites that sale those and other pond plants?

Thanks!