The Aquarium is open every day from 10am to 6pm. Cozy stands behind the counter while Chet and Chocola from Harvest Moon 2 wander the building. When you first visit the Aquarium Cozy will explain that he hardly has any fish in his tanks. He wants you to go out and collect fish for him, then bring them back.
Cozy has 16 fish tanks that he wants to fill, each with a different species of fish that live around the Island where your farm is. To give Cozy your fish, stand on the opposite side of the counter where he is, press the B Button to remove the fish from your rucksack, then press the A Button to throw the fish at Cozy. If it's one he doesn't have he'll thank you for the fish. If he has enough of the fish you're trying to give him, he'll let you know that he doesn't need anymore.
Each of the tanks will hold 1 to 6 fish of a specific species. You can view the fish you have caught by walking up to one of the tanks and pressing the A Button. You'll learn the name of the fish that occupies the tank and how many there are. Press the A Button again to select "yes" to look inside the tank or "no" to cancel. To end viewing just press the A or B Button.
If you stock all of Cozy's tanks then he'll reward you with a Power Berry.
Front Row
| Name | Picture | No. in Tank | Location |
| Snakehead |  | 3 | Forest |
| Black Bass |  | 3 | Farm & Forest |
| Carp |  | 2 | Farm |
| King Carp |  | 4 | Farm |
| Bluegill |  | 4 | Farm & Forest |
Middle Row
| Name | Picture | No. in Tank | Location |
| King of Lake |  | 1 | Forest |
| King Salmon |  | 2 | Forest |
| Pink Maomao |  | 4 | Village & Port |
| Blue Maomao |  | 4 | Village & Port |
| Parrotfish |  | 4 | Village & Port |
| Blowfish |  | 6 | Village & Port |
Back Row
| Name | Picture | No. in Tank | Location |
| Dorado |  | 3 | Ocean |
| Horned Shark |  | 3 | Ocean |
| Sailfish |  | 3 | Ocean |
| Archerfish |  | 4 | Ocean |
| Coelacanth |  | 3 | Ocean |