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[Ruth]

Well I have to say that the first thing that struck me is your fish load - IMO you have too many fish in that size tank. Naso tangs get very big and need at least a 6' tank.
You may have some success with a series of large water changes (at least 50% say 2 times a week) until the nitrates come down. be sure to use a turkey baster to give your live rock a good blast before water changes. A skimmer upgrade may help as well.
As has been stated if you have any sponges in your canister filter be sure to rinse them often. With that many fish you should probably rinse every other day with RO water.