1. http://www.dlink.org/aqua/image/misc1.jpg
2. http://www.dlink.org/aqua/image/misc2.jpg

Are these two shots of the same plant? It looks like it could be Hydrilla,
but they can be confused with species of Egeria, Elodea or Lagarosiphon.
Without a size reference, its hard to tell for sure from your photos.

First one's leaves are more like needles. Second one has flat and little
bigger leaves.

3. http://www.dlink.org/aqua/image/misc3.jpg

A species of Bacopa, probably B. caroliniana. An easy plant to grwo, will
flower (pale blue flowers) above the water's surface

I have a B.Caroliniaana. but these leaves are bigger than B.Caro.

4. http://www.dlink.org/aqua/image/misc4.jpg

Blurry photo, but probably a species of Limnophila. One of my favourite
stem
plants. Likes lots of nutrients (otherwise it quickly goes pale). Will
change leaf form and flower if allowed to grow above the water line.

Sorry. I changed the Picture now. I copied the wrong photo of that plant.

Thank You
Dominic