The Aquarium is a key item in Dark Cloud 2 and you are required to make it in Chapter 4.
For storyline purposes, you need it to get an Electric Fish, which you need to feed to Shingala, an infant Shigura that is under mind control, to short out the device controlling him. Osmond gives Maximilian and Monica an Electric Worm to feed to a fish inside Max's Aquarium, if you haven't made one before Chapter 4 (it is possible to make it in Chapter 1) he'll tell you the ideas needed to make an Aquarium, then he'll give you the Electric Worm.
- Fountain - Found in one of the two "town square" type areas in Palm Brinks.
- Window - Found on any building in Palm Brinks.
- Wooden Box - Found between two houses or on the corner near Cedric's Shop in Palm Brinks.