Personally, I'm a fan of Bliss Softening Socks... but we're going to talk about filter socks in this article. :)
Filter socks vary in size, material and filtration capabilities. The standard sizes available are 4 and 7 inch diameters.
The length of the socks varies by manufacturer. I generally prefer the longest sock to fit my 17 inch tall sump. A bigger sock can hold more detritus and filters your system longer than shorter socks. This is great for when I am traveling but also when I am lazy. I typically go 3-4 days before a short sock needs to be changed, and about a week before the longer one is clogged up.
Socks are made in felt material and mesh. The felt comes in 100 or 200 microns. Micron is short for micrometre which is one-millionth of a meter (I just Googled that). ;) Basically the smaller the micron rating of a filter sock, the more crap it will filter from your tank. So by that measurement, a 300 micron sock (usually the mesh kind) won't catch much except boulders. A 100 micron filter sock catches much more detritus and particles and will need to be changed faster than a 200.
And, as you probably guessed, the second kind of material available for socks is nylon mesh. I prefer to use the nylon mesh to catch flatworms that I siphon out in case of a population explosion. Been there, done that.
Another variety of filter sock available is the drawstring kind. They come in both felt and mesh material and are ideal for sumps which do not have a filter sock holder or for those that would not fit the standard sizes available.
How Dirty are your Socks?
Each tank is different and what your filter sock pulls out might surprise you. I've seen amphipods, asternia starfish, bits of algae and baby limpets by the dozen. Some aquarists have found a fish or two that took the trip down the overflow into the sock. Cleaning the socks is easy and it is best to have more than one sock around so that you can alternate without shutting down your filtration for lengthy time periods.
Some hobbyists prefer to turn the sock inside out, rinse very well (beat the sock against the side of the sink I say) and throw it back in. This preserves the biological filtration capacity, however, this should not be a concern in established aquarium. Hydrogen Peroxide is also used as an oxidizer of organic matter and to clean the sock. I found that it turned the sock yellow.
If you intend to throw the sock in your washing machine, run it without soap on just the rinse cycle and pray that your significant other is okay with it.
The method I use is bleach. My intent is not to bleach the sock to a pure white (and that never happens) but to most effectively get it clean of organics. I start with a dirty sock or two. I rotate between 3 socks that are 200 microns each.
I fill a small bucket with hot water and pour in a generous amount of bleach and let it soak for a minimum of 3 hours or overnight. Then I rinse the sock very well, inside out, repeatedly till all the grit is released. After that I soak the sock again for 24 hours with water and Seachem Prime.
Prime is a liquid that converts ammonia to a non toxic form and also removes chlorine that would be in the tap water I use to clean the sock. I rinse again and let it dry but, at this point, it is ready to be used again without it being dry. Any ammonia that could remain is food for the macros in my refugium and for my clams in the display. The bio-filter is more than adequate to deal with my method in case of a misstep.
Dirty socks are an important indicator of the filtration in your tank. Detritus that is suspended and sucked into the overflow to the sump is visible in the filter sock. If your socks are too clean and don't need changing too often, hopefully that is because you are running an ultra-low nutrient system by choice, and not because the crap is staying in your display.