Side quests and activities in Mass Effect 3 offer opportunities to earn XP, credits, and reputation points, as well as acquire unique collectibles. These missions often have specific availability windows, with some becoming inaccessible after certain main plot missions are completed, particularly before Priority: Cerberus Headquarters.
Collectibles, such as model ships and fish, can be purchased from various stores and displayed in Commander Shepard's room aboard the Normandy. These items are purely decorative and provide no other in-game benefits.
Fish Collection
Fish can be collected and displayed in Commander Shepard's aquarium on the Normandy. If Kelly Chambers survived Mass Effect 2 and is met again in Mass Effect 3, she will return any fish she was caring for. An Aquarium VI upgrade module can be purchased from the Elkoss Combine Arsenal Supplies terminal in the Presidium Commons on the Citadel for 25,000 Credits. This module automatically feeds and cares for the fish.
Side Mission Example
During the mission N7: Cerberus Attack, you can find datapads in the control room. Grab these before finishing the mission and give them to Alison outside Purgatory. Completing this side quest rewards you with 30 XP, 1,000 Credits, +5 Reputation Points, and an improved War Asset - Alliance Fifth Fleet.
Note: No side missions can be completed once you begin Act III, so finish anything you want to complete before heading to Cerberus Headquarters. Some side missions are progress-sensitive or timed, meaning they will fail if not completed before a certain number of other missions are finished or specific main plot missions are reached.