Well, we all know about bio slime (it's wonderful points and some of it's clogging potential.)
There are many other things that cause cloggs;
gravel, perlite, other media
Fish crap and rotting food
leaves and other stuff falling into system.
Plant roots
Even heard of frogs/toads blocking up overflows
I've found a new (at least I hadn't heard of it before) potential clog issue. SNAILS!
I just cleaned out the trap basket on my pump this morning at it was covered in small snails. A week or so ago I noticed lots of small snails on the walls and any pipes in my big tank and it appears they migrated or were sucked into the pump intake plumbing too. I would guess I'll need to put some effort into clearing other plumbing lines at some point. Hopefully not that I've put a couple catfish into the big tank (outside the tilapia cage) they will help clean out some of the snails. I've never noticed any of the snails in the catfish tank in any location that the catfish can access.
What are some other odd items people have found clogging things up
