Money is the main item of insaniquarium.Money is needed to do just about anything. From buying Guppies to getting egg pieces money is the source of well, almost everything.
In Virtual Tank mode and the bonus levels in Adventure, shells are a different form of currency used for buying fish for the virtual tank and extras.
Grey Shell: 1
Gold Shell: 2
Blue Conch: 5
Niko's Pearl/Silver Spiral Shell: 10
Bag of Shell: 20