filter sock?

[Reef Pilot]

I used to use socks, but not anymore,... replaced by a big sponge between baffles. Got tired of cleaning them. Sponge is much easier and quicker, and does the same job.

Did try going with nothing for a while, but that was definitely not good. Water not as clear, and skimmer needed cleaning more often, esp the neck, as it would accumulate crap more quickly.