Sylvester is the first alien that encountered in Adventure Mode.


Sylvester has a dark blue color, has no visible eyes, has 5 pointed fangs, 3 finger claws and 2 glove blades.

Adventure Mode

Sylvester is the easiest alien in the game. He invades the player's aquarium in Tank 1 and Tank 2 and can only eat one fish at a time. He will get "full" before eating another fish.


"Sylvester used to run a popular sushi bar in downtown San Francisco, but when he discovered a mysterious wormhole that led directly to the Insaniquarium, Sylvester sold the restaurant to pursue his lifelong dream: EATING ALL YOUR FISH!"



As your first opponent that you encounter, Sylvester has no special abilities. Just keep shooting him until you are awarded a diamond. When you see him with Balrog, try to kill him quickly as Sylvester has low health.


Tank 1 levels 2-3, tank 2 level 1: Coax him to the bottom corner and spam-click.

Tank 1 level 5: Take out Balrog, and let an attacking pet deal with Sylvester. If you didn't bring an attacking pet, go for Balrog first to get his dangerous appetite out of the way.

Challenge Mode

Sylvester and Balrog are one of the greatest threats when in multiple signatures and over 2 clones. It is very difficult to keep all your fish from being eaten by them. You must have a fully upgraded laser gun or Gash/Rufus and Presto (as an attacking pet) for easier disposal of these aliens. You can also try to navigate all of the clones to the bottom corner to deal with them all at the same time.

Virtual Tank

Main article: Mini-Sylvester

Mini-Sylvester can be bought in Virtual Tank and grow into a full-sized Sylvester. Sylvester eats ultravores and drops Bags O' Shells, making him a useful curiosity in the Virtual Tank.


  • Sylvester is the first alien that you encounter.
  • Sylvester and Balrog are the only aliens that make the same roar when they appear.
    • Balrog is also the only alien Sylvester might spawn with, not to mention by itself. As such, if they are both spawned in Tank 1-5 or Challenge Mode, the roars will blend into each other.
  • Sylvester is the only alien that has no eyes; rather, its head has only a mouth.
  • Sylvester bears a resemblance to a xenomorph.
  • Mini-Sylvesters can only eat Pets and Carnivores.
  • Cyrax's blue tentacles may hint at how Mini-Sylvesters are spawned.
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