You can run Phosban and Black Diamond carbon or Renew in the canister. They might need to be in media bags depending on the make of the filter. Maybe a pouch of Purigen, too. Floss and filter mats are bad news.
You're at about 10 times flow now. Not terrible, depending on what you want to keep. I'd probably add a Maxijet 900 with a Hydor Flo aimed front and center. Put the other powerhead in one corner aimed to the middle of the front and the canister output in the other corner with the intake in the middle. That should keep the water moving good and eliminate most, if not all, dead spots. The Flo will give you some chaotic movement to help keep detritus suspended and moving.
The media basket and the surface skimmer basket are part of the "Deluxe" package that Red Sea offers for the Prizm. About $25 online if you don't have it already. Do you have the Pro version?
The longer the skimmer breaks in, the quieter it will become. And the higher the throughput, the less cavitation or leaked air it will have. The extra flow seems to pull the excess air along with it.
Crushed coral is known for trapping detritus and nutrients. The result: Endless Cyano Nightmare and/or Hair Algae Farm. Sealing it in with a layer of sand is good in theory, but bad in practice. The result: Nitrates Thru the Roof. Too much infusion of nutrients and oxygen into what should be an anerobic and low nutrient zone for denitrifying bacteria. The voids between the large grains of CC allow for too much water flow thru your sandbed, especially at the deepest levels.
You're gonna hate me now, but thank me later. Pull the rock. Scoop out the sand down to the layer of CC. Remove the CC and put it in your garden or flowerbeds. Put down a fresh layer of oolitic sand and put the old sand back on top. Total sand depth should be about 2 to 2.5 inches. Reaquascape.
I'm not sure, but it mught be easier to drain the tank while you do this to avoid having to work in some really cloudy water. If you leave the water in, you might want to keep the canister running and pack it with some filter floss to help clear the finer particles.
You'll probably need to drain enough water out of the tank into a large enough container to hold the live rock anyway. You might want to move the heater and powerhead to that container as well if you want to store the rock there until the water clears enough to allow you to see while you reaquascape.
It sounds like the tank isn't too far along to do this without a big impact. You might see a mini cycle. Just some friendly advice.
Definitely raise the skimmer and the flow.
