If you look at your tank, are there,any rocks or other decor that could cause dead spots in water movement and allow detrius to build up?. Do you gravel clean during water changes? A good way to check if your substrate is overly dirty is to waft your hand around back and forth a couple of inches above the substrate. If alot of detrius moves up into the water then you're not cleaning properly. If this debris you're referring to is fine particles you may need to add a polishing media to your filter. I favour pond filter mats. They're quite compact in thickness, easily cut to shape and inexpensive. These will help remove suspended particles,which might otherwise pass through coarser media. As said,you should never replace more than 30% biological media at a time, never all of it as your filter will have to cycle again. There's mixed thoughts on gravel cleaning. Some think it removes too much beneficial bacteria. However, excessive waste matter laying around has much more of a negative impact than any amount of gravel cleaning could do. If you do it systematically, you need only move around,in small areas cleaning to a depth of 20mm or so. The only time you wouldn't clean is in a heavily planted tank where its normal to leave it.
Another thing that makes life,easier is to have a "feeding zone". Not,good for,everyone, but in my,discus tank, fully planted all bar a 10" square. I feed mainly sinking foods, beefheart, frozen bloodworm etc, and it makes cleaning up up after them so much easier. Once a month I use my eheim gravel vac amongst the plants to avoid an overly heavy build up.
