CONTAINS MEGA SPOILERS, AND YES, I KNOW THAT WAS FAIRLY OBVIOUS AND PREDICTABLE, BUT IT'S APPROPRIATE GIVEN THE OBVIOUS AND PREDICTABLE MOVIE
You know that cheerful B-movie aesthetic that really rotten movies think they can employ to get away with being really rotten: "So Bad It's Good"? It's a fiction. It's either good or it's not. A generally bad movie can sometimes be fun, in a Lifeforce kind of a way - a film can be entertaining despite poor writing, acting etc - if done with enough verve, panache or style, if done in an interesting or imaginative way or even, occasionally, if it's just smothered in gory effects. But in those cases the things that redeem the movie - nice ideas, offbeat camera angles, gloopy splatter - are actually Good Things. No-one talks about So Good It's Bad: quality is not a continuous spectrum where you travel so far to one end that you emerge at the other.
But there's a school of cinema that thinks you can actually get away with very low standards if it's done with a wink and a smirk: that acknowledging a film's shoddiness excuses the makers from bothering to do any better. Troma used to do this (they probably still do but the films were so utterly tedious and tiresome I haven't looked in the last five years at least) and now The Asylum are carrying on the tradition, albeit without the childish taboo-busting, by making dozens of hopelessly cheap rehashes of current and forthcoming blockbusters (Snakes On A Train, Transmorphers, Paranormal Entity) or idiotic monster flicks with CG effects that would have been considered ropey in Sylvester McCoy's tenure as Doctor Who.
Inexplicably, Mega Shark Vs Giant Octopus was some kind of success, so where else can they go but Mega Piranha? In the Orinoco River in Venezuela, genetic scientists have created mutant piranha that are doubling in size every few hours until they're the size of a horse and able to launch themselves into third-floor windows. (In a nod to MSvGO's casting of 80s teen pop star Debbie Gibson, here we have the science team led by 80s teen pop star Tiffany who, sadly, has not weathered the intervening years as comfortably.) A tough US mercenary type is shipped in to find out what's going on, but the local military aren't having any American imperialist interlopers telling them how to run their country, so the piranha double and quadruple in size and strength until they can take down a battleship and even a nuclear strike doens't have the desired effect....
Obviously it's the most atrocious twaddle and doesn't even get by on the Golden Turkey shoddiness it's hoping for (and which we've already established is a myth and the last resort of filmmakers who can't make halfway decent films). The effects are abysmal, acting and dialogue are terrible and even at 88 minutes it drags. Look: if enough people tell you it's a lousy movie, it probably is a lousy movie. If the makers are all but telling you it's a lousy movie, why inflict it upon yourself?