Treasure chests are worth 2000$. It has the highest currency value in Adventure Mode. There are two ways to get Treasure chests.
Ultravore
Ultravores are available in Tank 4. They naturally drop treasure chests.
Brinkley
Feeding Brinkley is another way to earn money in Insaniquarium. He follows a cycle, and when fed the 108th time, he will give you a treasure chest in exchange.
Virtual Tank
In the virtual tank, these are replaced by Bag o' Shells, worth 20 each.