In reality the Xiphactinus was a saltwater fish. As seen in Proof, the Terra Nova colony seems to be located not far from an ocean.
The real Xiphactinus was also much larger than the ones seen in the market, suggesting that they are probably juveniles.
The head of a real Xiphactinus can be seen on this page. It has a protruding lower jaw and sharp teeth similar to what's seen on the show, although the snout of the fish on the show appears more rounded, similar to a piranha. The bodies of the ones hanging in the market are also much shorter proportionally than the body on fossils of adults, again more similar to a piranha.
Fossils of larval or juvenile Xiphactinus have never been found by paleontologists, so one could speculate that the juveniles were more piranha-like.