You are a goldfish on adventure in a place with all kinds of danger lurking at you. Collect 10 plastic yellow duck toys to buy weapons and ammo. There are a lot of different fish swimming around here: green zombifish, red bulletproof fish and grey fish that are some sort of test dummy which you can shoot as well. Make sure you don't get contaminated by dangerous fish, because that means you'll have to go look for an antidote.
Move with the arrow keys. Use the S to shoot, the A for automatic fire and P to pause the game or look at the instructions.