Change the filter media
Increase circulation
suck up as much of the detritus as you can
do a water change
run fresh carbon
reduce feedings
add a cleanup crew
Most important is water change and increase the circulation so that the detritus can be picked up by your mechanical filtration, then you can change that as needed.
Thomas
Do you have a cleaning crew? You need snails and hermit crabs. Other critters that help are detrivore sea cucumbers, brittle stars, sand-sifting gobies.