I'm New here and want some help....

[Oddball~]

Its just name given to certain fish because of their shark-like dorsal fin. A bullshark can be a true freshwater fish, though. Sharks are fish, so it's all one in the same.

"The picture looks a LOT like irridescent catfish" We said it was.

And a

 

[chinnp]

Props to you for doing your research first. Unfortunately most freshwater "sharks" (as I'm sure you've found out) are catfish and tend to get pretty big. They're better suited for a tropical pond than anything. There are lots of other cats that are not tankbusters. They come in all sizes and personalities too.