I don't know what your maintenance schedule is, but for me, I got my nutrient levels down by doing water changes once a week. I start by cleaning the glass, then blow any detritus, and left over fish food, off the rock using a turkey baster. Then start syphoning out the water. While syphoning I gently dab the hose all over the top of the sand bed to suck up any waste. I do not advise disturbing the sand bed to much, hopefully you have a good cleanup crew and critters that will keep the sand clean for you. I will also clean any sponge filter, filter floss, sock, you may have. I notice you have a canister filter to, those things can become nutrient sinks. so make sure that is cleaned out regularly to.The arrow crab passed for unknown reasons. Other crabs are still alive. Also I am having trouble with nutrient levels being to high I have started to feed only once a day instead of twice. Any thing else I can do?
Another thing you could do is add more diversity to your tank. I don't know if they sell them in LFS's but i put a bunch of bivalves in my tank that i scraped off some rocks on the shoreline. They are great at filtering water.
I hope this helps.