Fish gills, I am not sure. I would suggest you
use crab gills instead. The reason is because crab
gills are made of chitin, which won't rot
away as quickly as what fish gills are made of.
You then want to use a microscope to measure
the distance between the hairs of the gills. This
distance is what the size of the particles would
be that would clog up the gills.
I know you could get a better answer of what
actually causes problems by using live fish, but
you would be killing the fish in the process (and
in a very painful way). Are you planning to
publish your work? If you are, then great (and
I hope you are collaborating with a professional
lab to help you write the manuscript and so on).
If not, however, then I think you should avoid
using live animals in your experiment, for ethical