Crud in over flow

[xabo]

Have build up of crud in the overflows.....anything to worry about other than water displacement?

[WheresTheReef]

I would just use a gravel vac to siphon it out during your next water change.

[DaJMasta]

+1 it's usually a really loose brown sludge that accumulates in places with very low flow (I had it at the bottom of the rear chambers in my AIO.)  Haven't noticed particular creatures that seem to thrive in it, so I think it's pretty safe to just siphon out - I have on several occasions to no ill effects - but it does stir up easily so turn off the flow or prepare for some cloudiness.