High Nitrates

Great advice from everyone, can't think of anything to add.
stop the feeding, raise ph 8.2-8.3 water change, add more rock, should be 1 1/2 to 2 pounds per gal.

 

Welcome to the reef.. You've come to the right place for help.

Nitrates:
As stated above less feeding
How big are the larger fish? (that could be a lot of poop)
Consider a refugium to maintain balance PH..and the macroalgae will soak up phosphates and nitrates.
Personally I would do several 15% water changes. Not too much at once it can shock the fish with large changes in PH ect.
I would ad more live rock... maybe another 50lbs.

Got a pic of your filtration and tank?

 

As stated below, your No3 problem is ;

1. Over feeding, I mean waayyyyyyyyy too much.
2. When you changed out the substrate, you started a small cycle. Remember that what gets "cycled" is the LS and the LR. The LR and LS is what filters the system, by changing the sand, you destroyed half of the filtering system.

Solutions;

1. Cut down to feeding every other day. I know ppl who have Anthias and feed every other day and they are fine. You'll have to watch the Anthias.

2. If possible, have your LFS hold on to the livestock while you get through the mini-cycle. WC's will not fix this problem, in fact, it will only prolong the cycle. It will, however, lower your No3 for the time being, but only to go back up again. Luna