In the tank, I have about 110-130 pounds of live rock, and 2" deep of sand.
In the sump, I have a ASM Skimmer along with purigen in the first slot (where my overflow flows into) then that goes into LR
rubble, then into my sponge, then from my sponge into an empty slot with my return.
The only reason I want to remove the sponge is because I am fairly certain that is the source of my nutrient buildup. When doing water changes, I siphon any and all detrius on live rock and sand. I turkey baste the rocks thoroughly to get the stubborn stuff that doesn't want to come off.
I do use RO
water as well. I feed either mysis shrimp, garlic omega one pellets or omega one flakes once every 2-3 days only what the fish eat.
I don't have a phosphate test but I know I have them because I have been getting little slurges of GHA.
I was considering your idea of cutting a piece of sponge off each week, but I keep reading that sponges may do more harm than good.