The Fish Tank appears after completing one bundle. These bundles are completed by collecting [[Fish|fish]]. This requires catching fish with a [[Rod]] or [[Crab Pot]]. Different fish can be caught in lakes, rivers, and oceans, as well as different times of day and year.
Note: It may not be possible to finish the Specialty Fish Bundle before finishing the entire Vault section, as you need access to the [[Desert]] to get a [[Sandfish]], unless it becomes available at the [[Traveling Cart]].
Completing the Fish Tank bundles removes the Glittering Boulder next to the [[mines|mine]] entrance. [[Willy]] will give the player a [[Copper Pan]] which can be used to collect [[Blacksmith|ore]] from lakes and rivers.