from the looks of your livestock and what you have posted, it seems that your tank is not able to keep up with the bio-load, not to nagg, but the yellow tang needs a min. of 100 gallons to thrive, and im not sure how many gallons your butterfly needs but i know for a fact that a 55 is to small for it, i have a 60 and butterflys are a no-go for that size.
i would no be worried about your 'trates, i would try to do a water change to fix that ammonia level, with those fish in your 55 gallon you need to get those down, how long has your tank been running?