LOL, what ... you think I replace my prefilter when it's clean?
It won't hurt a thing if they're dirty and yes, I reuse mine, over and over. If it really bothers you, shake the dust out and give it a rinse in a bucket of water.
And speaking of water, our city water is terrible here for plants, so I started off using RO, against the advice of supersoil sellers. Now I know why. To many Cal and Mag problems, so now I run my city water through a double hose filter like Buggy Blue and GroGreen to remove the chloramine, then pH down it with lemon juice. E-Z P-Z!
I'm also looking into BTI (Bacillus thuringiensis subsp. israelensis) Mosquito Bits to kill the pesky little things, but for now the filter barier is working well. Only the occasional gnat, and the picture above shows how moist I 'try' keeping the soil at the top of the pots.
Hope that helps.