I know there is a reason for skimmer to do this but what is it?

[naclh2o nut]

I have a seaclone 150 and it is acting a little odd. It will produce about a 1/4 cup of the blackest gunk at night and nothing during the day? The foam comes up and looks like it is trying to push the cap off after lights out, then sometime around 8 am it just falls back into chamber and there is nothing I can do to get it back up. Lights go off around 11 pm and back on around 10 am.
I have a lit bio load-
3- blue chromis 1"-2"
1- yello tang 3"-4"
2- clowns 3"
1- goby 4"
100- polys
2- little frags of a colt?
3- shrooms
This is in a 90 gal, 20 gal sump, 200lbs L/R
Help explain please.

 

[scsinet]

Water chemistry changes when the lights go out due to changes in photosynthetic organisms.
THe skimmer doing most of it's "dirty work" at night is not at all unusual.

 

[spanko]

WOW! Learn something new everyday! Thanx Quint!

 

[naclh2o nut]

Thanks, just seems odd that it starts about lights out and stops before lights on.