Jeff,
First, welcome to the site! Have you lurked here before, or is this your first time in the pool? This is a great place to learn about the hobby....there are a lot of good people who are willing to lend a hand.
I guess the first thing that most people are going to mention is the size of the tank. Although your cycle may seem complete, a saltwater tank takes longer to mature to the point where a large bioload can be handled. 6 fish may not seem like a huge load, but believe me, it can be. Imagine putting 6 people in a 1-room cabin for an extended length of time, and telling them they can't leave. Even if it's a large cabin, people will get on each others nerves. Fish get the same way -- their territory shrinks, and they get stressed! Add to that stress a water chemistry that may be even
slightly sub-par, and you have an environment that can cause problems. If you added all 6 fish at once, or even within 2 weeks, the bacteria count in your tank (cycled or not) would not be large enough to handle the influx of new ammonia from the new fish. So you may be getting ammonia, nitrite, and nitrate fluctuations that you're not reading on your test sets.
Now, to the fish: Yellow Tangs are beautiful, and often very hardy animals, but they require MUCH more swimming room than a 46 gallon aquarium. In short, your fishie is going to outgrow your tank very quickly....if in fact it hasn't already! How big is the yellow tang now? Did you purchase one of the baby specimens, or did you get a larger one? The symptoms you describe sound like the beginning of a condition called HLLE, Head and Lateral Line Erosion. It's a condition brought on most often by....you guessed it....STRESS! I don't know for sure if this is a correct prognosis; someone else can help back me up or correct me I hope, but it sure sounds like it.
I'm sorry to say this, but you should probably return the Tang to the LFS. I'm not the Tang Police, and if you want to try and keep him you're welcome to, but just be aware that most tangs do very poorly in small aquaria. Even animals that seem fine may have shorter life spans...early death brought on by stress levels, outcompeting for food, swimming space, etc.
Don't feel bad, many of us get bad advice when we first start the hobby. Just to give you a personal example, I was similarly sucked into a yellow tang when I started the hobby. I had a 55 gallon tank at the time, and after (and I cringe at saying this) 4 weeks of cycle, the fish store that shall remain unnamed told me "sure, I can throw a coral beauty, yellow tang, 2 false perc clowns, and 2 damsels in..."
"All at once?" the naive young man asked....to which they replied,
"No Problem! Shall I ring that up?"
Needless to say, SOMEone had a tank crash a mere 2 weeks later. The only survivor of the ich that ravaged through my overstocked and underfiltered tank was a single false perc. clownfish. And yes, I still have him today, albiet in a bigger tank.
