Filter socks are there to polish the water, and really, that is about it.
If you want to just them for nitrates, you're going to have to replace them often, and ultimately, cleaning them will feel like a waste of time in the long run.
Im not saying that it isn't doable, but you'll want to just buy a pack of 10 or more so that you get a bulk discount, and then after you go through 8 (replace them once a week), throw them in the washing machine in a garment (bra) bag and wash them in hot water with a small amount of bleach with a rinse of warm and a rinse of cold.
You only get so much use out of a filter sock before it is permanent clogged, and you might get more life with a more intense wash with something like oxiclean, but that detergent still scares me. You could also try vinegar.
That being said. 100micron will filter better, but it will undoubtedly be a bigger headache, but for what? Slightly fewer nitrates?
Don't kill yourself. Go with the 200+