I used to have CC in my 55 and would stir it up regularly, after putting a clean cartridge in my canister filter, to clean up the gunk released from the CC. I changed to sand and it was the best move I have ever made in my tank.
Stirring up the CC can really release a whole bunch of bad stuff and can contribute to the nitrate levels you have reported. I know that some people swear by CC, but IMO it is a nasty substrate and recommend switching to sand.
On feeding, I feed my 55 gal reef tank with 5 fish only a couple of times a week and the fish are doing great. Less feeding can be better when it comes to feeding.