My suggestions to win the battle:
-weekly or twice-weekly water changes of 30%
-baste your rock and sandbed (baste hard into the sandbed) to get detritus into the water column where your skimmer, HOB filter with foams, and/or canister filter can remove the detritus and ALWAYS throughly clean the HOB and canister filters after basting. Basting can be done three times a day for a week for the best results. Your tank will look like a horrible snowstorm after a good basting, especially at the beginning, but it will not hurt your corals.
Detritus Export Video
After you get the situation under control, do weekly maintenance.