WASD = Move Mouse = Aim Left-Click = Shoot To complete a level, shoot the required number of fish. Make sure to keep an eye your oxygen (O2) levels, and to come up for air before you run out. You cannot shoot when you're taking a breath. Watch out for sharks and other tricky fish; they'll reduce your oxygen. When you've shot the required number of fish, come up for air; this will complete the level. Between levels, you will see how much money you earned. Your money earned is based on the number of fish you killed and the price of fish for the day. You can use your money to upgrade or buy weapons. You can also sell old weapons for 75% of the original price.