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 reasons.

You may want to start by researching the structures and sizes of fish gills. If you can read about how and where particles get stuck in fish gills, you can start to choose what sizes of particles you would want for your experiment. Good luck!

Note from ScienceLine coordinator:
You should avoid using live animals in your experiment, for ethical reasons.