Treasure Chests are worth $2000. It has the highest currency value in Adventure Mode. There are two ways to get Treasure Chests.
Ultravores are available in Tank 4. They naturally drop Treasure Chests.
Feeding Brinkley is another way to earn Money in Insaniquarium. He will produce some kind of Money after 3 feeds. Brinkley follows a cycle, and when fed the 108th time, he will give you a Treasure Chest in exchange for the food.