I only open up my fx5 every 6 months which is on my 125g...but, I also have on there an AC 110 which I clean out monthly and a Fluval U4...fluval submersible, in addition to a uv sterilzer w/powerhead. In the filters I have the regular foam, carbon, bioballs, purigen, etc
Lots of driftwood in the tank as well. Actually a lot, thus the purigen
I have so much filtration because of the overstock and increased feeding and poo as this is the grow out tank....
I just cleaned out the crud out of my sump after a year, and the overflows...
I don't stress too much over the crud. I mainly just look to see how the fish are doing, then decide if I want to tackle the filtration