Even though special effects had made considerable progress since 1978, stock-footage from Joe Dante's version was used for close-ups of the killing fish of that film.
Rather than shoot new special effects for the film, Roger Corman simply recycled the effects from the original. Sets and props were also designed to look like the first film to help accommodate the recycling of the special effects.
Despite being described as 'freezing', the pool that the piranhas were originally in had steam rising from it, indication that it was, in fact, heated.
When William Kat jumps into the water near the end of the movie..you can clearly see the markings on the swimming pool wall that they filmed the jump into.