You play as a first-time fish store owner, trying to grow your burgeoning business. To do this, you'll need to populate your tank with new fish, feed them, and sell them once they grow to adult size.

The game starts you off with five fish already in place. Clicking on a particular fish shows its current age, gender, and the type of food it wants to eat. You'll also see how hungry it is, its health, its current happiness, as well as its current monetary worth -- older fish sell for more than younger fish. The fish grow over time, and there is no way to speed the growth process.

At the top left of the screen, you'll see that level's goals. These typically fall along the lines of "Sell x number of fish" and "Earn $y", though later levels provide increasing challenges. To the top right of the screen is your current dollar holding, and beneath that is the time limit. If you can complete all of your tasks beneath a certain time, you unlock the Expert rank for that level, which gives you an item for your Trophy Tank, visible between levels, and the Sandbox, in which you can mix and match all of your unlocked items.

Your fish get hungry at regular intervals, and if you don't feed them regularly they'll die, requiring that you select them and click the "Flush Fish" button to remove them from the tank. Each tank can only support a given number of fish, so it's important to consider this when choosing which fish to grow in your tank, as some fish take up more space than others. Fish let you know they're hungry with a little pop-up balloon above their heads; to feed a fish, you'll click on the "Supplies" tab in the bottom panel, and select "Food". You'll see a list of available food options, along with their list price. Click once on the appropriate food, click again in the tank to drop the food. There is always just enough food per drop for each fish type that eats it; all other fish ignore that type. Food types include flake, brine shrimp, blood worm, and pellet.
At regular intervals, algae will show up on the tank. At the early levels, algae doesn't affect your fishes' health, but items can't be selected if they're behind algae, so it's worth taking the time to remove it. You do this by clicking on the "Tank Status" tab, where you'll find the algae scrubber. Click on the scrubber to select it, and simply put the scrubber over the patch of algae to remove it; no rubbing necessary.