Nice fishtank...no fishies........

they better show up soon or this topic will go downhill...
This is a long exposure, so they were blurred out...
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The Big Stopper is a ND3 filter (I think) ... see here for more info about ND filters
Neutral density filter - Wikipedia, the free encyclopedia
thus with it making the image much more darker, you have to increase your shutter speed. As you increase your shutter speed things get blurry .. as you increase it more, things start turning into moving ghosts .. and more .. they disappear unless they were not moving.
you can experiment a bit without an ND filter by using a smaller opening aperture. Try F/8 then f/11, f/22 and notice how your shutter speed times slooooow way down and then you can get motion, blurry etc.