I plan on going to the dollar store to buy various kinds of bio media.
I want to put this in mesh bags of some sort to make it easier to
clean. Will the red mesh bags that you buy onions in at the grocery
store work? I don't want to have a pond full of red fibers next month.

The two 'tests' I can think of are to burn it and see if it burns like
plastic, and boil it and see if it decomposes.

HTH

PS I've read that some plastic kitchen scrubbers have mold inhibitors in/on
them so you might want to avoid them. I'm thinking of using plastic fencing
in my new filter.