I use 200, 100, and 50 micron socks. The 200 does a pretty good job with clarity and for the most part is what I use since I get three or more days 'tween service intervals.
The 100 and 50 do give better water clarity but in my case not that great a difference and they need to be changed twice a week with our bio load and feeding habits.
Washed. The basic type felt socks can last me 6 - 7 months before they are trashed and are very easy to clean. Just take it outside and give it a go with the hose, let it soak in some dilluted bleach for a bit, rinse well and its ready for reuse. Some toss em in the washing machine.
Geeze, I thought I would get a few more opinions than this.
Would it be safe to assume that most sump users don't know the answer ?
Or is the question difficult to answer because the filter media being used is ungraded ?
At the moment I use 4" coarse sponge on the flow in to the sump and then I use 4" thick layer of fine sponge and a 2" layer of fine floss.
I am changing this around though and it will be 4" fine sponge with a 200 micron pad on top at the inflow and then 8" of fine sponge and a 100 then 50 micron pad on top. The 100 micron is not definite. That should see me fine for mechanical filtration.