This is a paid asset, but now you can download Fish School Bundle Free.
Detail this asset from Unity Store: Original Link
Fish School Bundle v1.3.3 (Latest version)
Fish School perfect for underwater scenes. This asset is very simple to use, simply drag the prefab to the stage. Commented inspector editor will assist in achieving the desired result.
• Drag & Drop Fish School
• Gizmos that shows roaming area
• Simple UV map, easy to change textures
• Easily customize
• Easy to add custom animated models
• Includes Bubbles, Shaders and Demo Scenes
• C# Scripts
• Distance Disable Script
• Obstacle Avoidance Steer and push away from colliders (No Rigidbodies)
★ Fish School Bass
★ Fish School Salmon
★ Fish School Goldfish
★ Fish School Koi
★ Fish School Amberjack
Combined value $90~
Unity 2019(+) compatible
Support available for assistance if any problems should occur.