Look into Spiny waternymph and Spiny Naiad. I don't know how to eradicate it, but knowing what it is can be half the battle.

I merely did a internet search for you by typing in "saw toothed pond weed" (a simple descriptive phrase)and looked at the image results, found some that looked like yours and visited a few sites and gathered a few names from official looking botany sites. Please verify my suggestions before spending any money on weed killer.

Best of luck!

Fish on!,
Noel