i dont really know alot about these fish except there BIG and mostly mean.....i will share with you what i do know though....

Clown Knife Fish
this fish just looks plain old cool......

Photo by Chad Christansen from MASI
Spotted Gar

photo by Nature's Images Inc.
Longnose Gar

photo by Garold Sneegas
Alligator Gar

photo by Nature's Images Inc.
 

Pacu

Giant Gouramis

Sturgeons

smaller aquarium sized Shovelnosed Sturgeon

photo by Nature's Images Inc.
Plecostamus

Tinfoil Barb

Nile Perch

Snake Heads

African Tiger Fish

 photo from Lurking in the Weed's LTD
South American Tiger Fish
look at those teeth..........

 photo by The University of Michigan from LOS PECES PIRANHAS Y CARIBES DE SUR AMERICA
Siamese Tiger Fish

Candiru
ugly bugger hey..... apparently the larvae of this fish can swim up a persons urinal tract and wedge itself inside u..... ouch....

photo by The University of Michigan from LOS PECES PIRANHAS Y CARIBES DE SUR AMERICA
Bowfins

Mud Characin

 photo by The University of Michigan from LOS PECES PIRANHAS Y CARIBES DE SUR AMERICA
 Paddlefish

Sawfish
what a monster............ i want one ......<grin>
this isnt really a aquarium species..apart from maybe a huge public one....and the chances of you the hobbiest getting one is vertually nil..... and with that the Saw fish isnt really a freshwater fish.....it lives and breeds in salty water...but has been found over 400km inland.. it is also a close realtive of the shark...

Background Photo by Chad Christansen from MASI