If you have any ro/di water left from the lfs test it for nitrates, you may be adding more to the tank, also 3 times a day feeding is way to much, cut this down to once every other day for now untill you get reduced nitrates, and using a external cannister will not help they are nitrate factorys, clean the filters out or even replace them at least once a week, would also suggest doing some back to back water changes, hope this helps ....
One other thing if you say the lfs has tested the water then ok, thought your test kits may of been out of date???
Stingray... _________ Hardware-55G Rena Display,Filstar XP3 Canister Filter, 200w Rena Heater, 1xKoralia 3, 1xKoralia 2, Lee's Medium Airline Protein Skimmer, 8x39watt T5 Fish-Yellow Tang, Midas Blenny, 2xClowns, Neon Damsel, Blue Yellow Tailed Damssel, Chalk Goby Inverts-Snails, Hermits, Black Long Spine Sea Urchin, Cleaner Shrimp, Coral Banded Shrimp, Feather Duster, Green Bubble Tip Anemone, Coral's-Green/Purple Mushroom's, Waveing Xenia, Button Polyps, Green Star Polyps,  |