Simply type these codes in during a level to gain the described effect:

space - The tank's background will look like space
supermegaultra - Changes Preggo's birth noise
time - Toggles the timer in the bottom-right corner
void - The tank's background is now blank/white
wavy - Waves will appear in the tank's background
welovebetatesters - Changes breeder fish's birth noise
zombie - All of your fish will look dead

Silver Trophy - Get all four bonus pets, +1 pet limit and +4 virtual tank pet limit
Gold Trophy - Unlock all stories from Challenge Mode
Sandbox Mode - After unlocking the silver trophy, go to the main menu and press Up, Up, Down, Down, Left, Right, Left, Right, B, A

In the virtual tank, buy a regular guppy and name it 'Santa'. The fish will become white with red fins; grow quicker than usual; have a Christmas-related biography and play Christmas songs every now and then.