Simplified Electrical Impedance Tomography (EIT) setup
Neat problem.
Unfortunately, water-ish things with dissolved ions are quite conductive.  If the diameter of the object is 30-40x smaller than diameter of the tube, I sincerely doubt you're going to be able to detect any significant resistance change as the water will be significantly more conductive than most things you pass through it.  And, if you're talking metal, the drop in resistance over the small area is swamped by the larger resistance to get the current to the area.
Given that the stuff you are talking about seems to sink in water, this indicates that you've got something with a density quite a bit different from water.  I would probably try to use sound/ultrasonic transducers of some form.  Either put the transducer at the bottom/top of the tube and bounce sound up/down (looking for reflections and possibly a Doppler shift) or bounce the sound across the tube and look for reflection time changes to indicate a density change.
I'm thinking something along the lines of an ultrasonic anemometer but for liquids.
Good luck.  Post back with whatever solution you come up with.