awesome bits of kit the skims, just be sure to wash the sponge out every other day! Scum is very unpredictable and can appear out of the blue on a mature healthy tank. Strangely enough it seems to disappear on mine when im lazy and dont WC, clean filters etc... im not advocating that as a solution in any way though!!!
I got bored trying to figure it out so bought skims for all tanks that dont have dwarf shrimp (as it eats them)
i also believe that it can contribute to BBA by messing with gas exchange, everytime scum appeared on mine BBA would pop up a week or two later.... of course i can hear clive telling me something about cause and effect with this statement