In the game, aliens will appear periodically, trying to attack, eat, or kill your fish. All you can do against them (aside from Gus) is shoot them with a laser gun.
The original game, only entitled "Insaniquarium", only featured four aliens.
Main article: Sylvester
Sylvester is a blue alien with no eyes, has 3 white claws on both hands, and has a tail instead of feet. He is the first alien ever to be encountered in the game. He eats your fish, but can only eat one at a time before he gets "full". He is the weakest alien in the game.
Main article: Balrog
Balrog is an orange alien with red hair, red eyes, has fins on the sides of his head, a muscular body frame, 3 claws on both hands, and a tail. He is the second alien encountered in the game. Unlike Sylvester, he doesn't get full. He practically resembles that of a lion.
Main article: Gus
Gus, as described by the game, is a baby-faced man child, and has a fish tail. He is special from the other aliens as he cannot be defeated using lasers. Instead, you defeat him by feeding him excessively until he bursts. Anything edible can damage him, your fishes, fish food and even star potions. He is the third alien encoutered in the game.
Main article: Destructor
Destructor is a robot with green eyes, spiky "teeth", and fires missiles at your fish. He doesn't directly eat your fish but uses fish-seeking missiles to kill them. He can only move around the bottom of the tank. He is the fourth alien encoutered in the game.
After the release of the Deluxe Version, which was now entitled "Insaniquarium Deluxe", the game featured four new aliens.
Main article: Ulysses
Ulysses is a stone cyclops which fires energy balls at your fish. Almost the same as Destructor, but more deadly. You see, when the energy ball is blasted by your laser, it's deflected and becomes a threat not just to the targeted fish, but to the entire tank! But if done right, the energy ball can be deflected back at Ulysses, damaging him.
Main article: Psychosquid
Psychosquid is just what the name says, a psycho squid who loves to eat your fish. He apparently holds a butcher's knife, a fly swatter, and a fork. He is not as easy as the other aliens, who is simply defeated by being blasted repeatedly. After some time, Psychosquid will turn blue and drop down his tentacles. Beware not to fire at him, because instead of damaging him, you help regenerate his health! He is a serious threat especially if you don't know how to battle him.
He technically is an octopus.
Main article: Bilaterus
Bilaterus is a two headed skeletal alien. Apparently, you have to battle both of them. Don't worry, not both at once. You have to beat one before proceeding to the other. Easy, right? That is if you can defeat him fast enough. If any fish touches any part of his body, yes, any part, well, bye-bye! Bilaterus can be a more dangerous foe than Psychosquid if not handled correctly.
Main article: Cyrax
Do you dare face the evil alien mastermind, Cyrax? Well, of course you do! You need that egg! Or, shells... Don't expect it to be an easy fight, though. He can summon Mini-Sylvesters and any alien you encountered before (not all at once, except for Sylvester and Balrog, who both get summoned together)! He'll need about 50 hits before he goes down. He only appears in the Final Boss Fight.