C, and once per week. I use a spatula (my spatula, not one from the kitchen
) and wipe the mud out of the reaction chamber. The riser and collection cup I just wash out at the sink.
Then I take a paper towel and wipe down whatever I missed.
I try to make a weekly occurence, otherwise the skimmer loses effectiveness. Just because I try to make it weekly though, doesn't mean that it really is.