Black "gunk" In Fluval C-4 Filter

[Msradell]

The filter pad in our Fluval C-4 filter is constantly getting plugged with a black gooey buildup.  We put a new pad in 2 days ago and it's already plugged tonight.  We are you running this on a 29 gallon freshwater tank with a normal population of fish, gravel bottom and artificial plants.  All of the water parameters are very good, 0 ammonia, 0 nitrates, 20 nitrates, 120 GH and a pH of 7.2.  We are also noticing some buildup in the gravel.  We have had on occasion some black algae in the tank but that has been pretty much under control recently but there is a mildew smell in the tank.  We are also running a 7 W UV filter in the tank.  All of our fish seem healthy but we're going broke replacing filter pads!  Any idea what this material we are getting in the filter is??
 
Thanks,
Mark

 

[N0body Of The Goat]

Define "normal population of fish!" Species, numbers, rough size of body excluding tail fin ("Standard length.")
 
Dimensions of this 29g?
 
You should not be continually replacing filter pads, I'm still using the same ones I started with upto four years ago. When these get clogged up, slowing down filter flow, give them a squeeze/rinse in removed tank water.
 
Pads will get clogged up with decomposed food that the fish missed; fish faeces; "sludge" produced by the beneficial bacteria that keep the water safe for the fish; bogwood that is rapidly breaking down etc.

 

[Msradell]

It's a standard 29 tall tank, 30" long, 12" front to back and 18" deep.  The fish population is: 1 twig catfish that is full-grown, 1 full-grown bristle nose pleco, 4 neon tetras, 4 balloon belly mollies, 3 clouds, 1 Emerald cory 3", 2 peppered corys 1" and 1 Julii Cory 1".  I used be able to clean the pads very easily but whatever this is just doesn't rinse out!  I don't have any bogwood, just 1 piece of Malaysian Driftwood.