Give Fish behaviour Bullet.
Make 4 sprites as long lines, call it LeftBorder, TopBorder etc. Have it on the edge of your layout. Have it set INVISIBLE. These will form the boundaries for your fish.
Have a Collision check.
Fish Collision with TopBorder = set Fish angle random(30, 150). Whenever fish touch top border, it will swim downwards in a random manner, ie. it will change direction.
Do the same for collisions with other borders, just set the angle to be in the opposite direction with random.
Another thing you can do to make it even more random roaming:
Every Random(5,10) Seconds = Set fish angle random(0,360).
Try that and your little fishies will be swimming all around withing your layout.
If u want them to flee in the opposing direction when attack, just set their angle on an attack trigger and increase their bullet speed for a few seconds before returning it to normal speed.