Good question. This might be the reason for some high nitrates and phosphates.
If I were to clean mine which I probably need to do I would use a wet/dry vac with a long hose
wipe the sides down or scrape them, stir it up good, pull the over flow pvc out of bulk head and let it drain into sock. thats if you have the room in your sump for the extra water.
it maybe beneficial but at the same time at the very bottom stuff is building up that may cause high nitrates or phosphates.. my turkey baster does not fit to the bottom of my current setup so am trying to come up with an efficient way.. More people must have the same question or ways to clean it
I was told by a few on here to put the smallest powerhead one could find and put it in the overflow to keep the gunk from settling. I haven't done it because I did not have a free outlet but one just recently opened up which reminds me now to give it a try.
i took a spare power head last night and sprayed the inside of my boxes and stirred some stuff out into a sock but not much came out, one thing that did came out were about 10-12 tiny snails that i threw back in the tank