Anyone use tulle as a filter medium in a 'Skippy's Type" filter?

Tulle is used in dressmaking. Check
http://www.teuscherboston.com/images/wed05.jpg
to see what it looks like.

We've used it for two years and it seemed to work satisfactorily. We
have a few inches of black sludge on the bottom of the filter and are
told that there should be only sand and not sludge. Any ideas on this?
Is sludge OK or is it a sign of poor filtration?

What do you think? Continue using tulle or try something else readily
available. We spent about $60 CAN on huge bundle of tulle fabric for
our 200 gallon stock tank.

Many thanks........