The Sandbox mode can be accessed after getting either the Silver Trophy or the Gold Trophy. Entering the "Konami Code" : up, up, down, down, left, right, left, right, b , a, at the main menu screen, you will be instantly brought to the Sandbox Mode Tank.
Key Reference
While in the Sandbox Mode, you have the power to spawn any type of fish, pet or alien (except for Cyrax, though). Each has it's own corresponding key in the keyboard.
Fishes
Guppy - 1
Carnivore - 2
Star Fish - 3
Starcatcher - 4
Guppycruncher - 5
Beetlemuncher - 6
Breeder - 7
Ultravore - 8
Pets
Stinky - Q
Niko - W
Itchy - E
Prego - R
Zorf - T
Clyde - Y
Vert - U
Rufus - I
Meryl - O
Wadsworth - P
Seymour - A
Shrapnel - S
Gumbo - D
Blip - F
Rhubarb - G
Nimbus - H
Amp - J
Gash - K
Angie - L
Presto - ;
Bonus Pets
These must be in capital in order to spawn the bonus pets.
Brinkley - A
Nostradamus - S
Stanley - D
Walter - F
Aliens
Sylvester - Z
Balrog - X
Gus - C
Destructor - V
Ulysses - B
Psychosquid - N
Bilaterus - M
The tank background can be changed with the "+" and "-" keys.