No big danger, then: as sharks need saltwater, putting them in a pond would be their death sentence.
Bull sharks can live in fresh water. They will find you and kill you regardless of what water you live in.
I learn things every day from TrekBBS. (Okay, not really, but it seems like the thing to say.)
Wikipedia mentions two types of freshwater sharks
: bull sharks
and river sharks
. I find the bull shark particularly cool because itís a cannibal.