I am working on a sketch, based on the flocking algorithm from daniel shiffman. I extended the code to work in 3D. Thats all working fine, so far. I wanted to make a sketch with a swarm of fish, and fish don't swim with their stomachs up. My fish do. So i really can not figure out, how to rotate the model to avoid the "dead fish" behavior.

Thanks Thomas!

1.  PVector modelOrientation = new PVector(0, 0, 1);
       PVector heading=new PVector(vel.x, vel.y, vel.z);
       heading.normalize();
      
       float angle = acos(modelOrientation.dot(heading));
       PVector axis = modelOrientation.cross(heading);
       pushMatrix();
   
       translate(loc.x, loc.y, loc.z);
       rotate(angle, axis.x, axis.y, axis.z);
       flockFish.draw();
       popMatrix();