They are great for capturing free floating particles (depending on the micron of your filter sock), however, they do require maintenance or yes, they will become a nitrate factory just like foam blocks.
I always have a stockpile of extra filter socks on hand. As soon as I see the water having a problem making it's way through the sock or they turn a slight brown, boom, out they go. For me it was every 3 days on 7 inch socks.
Then to clean them I turned them inside out, gave them a quick rinse in the sink and then into the clothes washer with a cap full of bleach. I also set the washer to HOT and made sure the extra rinse cycle was on.