Using Neutral Density Filters for Longer Exposures

Photographing the fish in my backyard has been part of an on-going project for several months now.  Recently, I've been using longer exposures to capture the swirling movements and frenzied excitement of the fish while they feed. 

0.4 sec at f/11

But sometimes even f/22 at ISO 100 doesn't permit a slow enough shutter speed for the effect I'm after.  That's when a neutral density filter is called for.  Landscape photographers use it to blur the motion of water.  They mount the camera on a tripod and slow the shutter enough to produce a dreamy, almost milky effect on the water.  I hand hold the camera and track the fish while they swim around.  Or I twist the camera while the fish move about. The slow shutter speed can produce unexpected results.

 1.0 sec. at f/11

0.8 sec. at f/11