First, let me say that I'm sorry for your loss......Bannerfish are really spectacular fish, and definitely one of my favorites. I can definitely empathize with you there.
Unfortunately, Meowzer is right on the money with her assessment. I would not add any new fish to your tank.....it's almost certainly bound to be killed within weeks due to aggression. Look at it this way: your original Heniochus probably "got along" with the rest of the group because of the simple fact that they were all added within a certain period of time. Now that fish is gone, and the rest of the tankmates essentially expand their own personal little territories to take over whatever place the butterflyfish had staked out. This sort of territorial sorting-out doesn't take long -- within hours of the fish dying, in fact. Any new addition will be seen as an outsider competing for territory, and your current fish will simply harass it until it dies. Consider a large family in a small house. Let's say older son Johnny has a room by himself, and the younger brothers Jimmy and Jake bunk up in the only other room. Johnny goes away to college, and what do you think happens to his old room? It certainly doesn't stay empty for long, that's for sure.
Now, on to your other problem. The one you don't see, because your fish "have been fine" all this time. I'm sorry to say that your stock list - even minus the bannerfish - is far too big for a 55g tank. I'm not going to harp on you about tangs and tank footprints and all the rest, but I will give you the simple info: You need a bigger tank, or fewer fish. My vote is always going to be "bigger tank," of course. Let's look at your list:
Snowflake eel: Okay, fine. I'm not a fan of eels (never saw the cool-factor in them) but that's not your problem, it's mine. A snowflake eel shouldn't pose a problem for a well-filtered 55g tank. Well-filtered, I say, because even though they will live in a 55, they are messy animals that produce a lot of waste. From what I have read (again, no direct experience), most Eels are nitrate factories.
Medium size yellow tang: Pretty questionable. People have been keeping Yellow Tangs in 55g tanks for a long time, even though they probably shouldn't. You say "medium," but I have no way of really knowing what that means. YT's can get pretty large. I know there is an argument that a fish "won't get as big in captivity." You're right. It will die before it reaches full size. I'm not the tang police, and although I don't like the idea of keeping a YT in a 55 for the long term, I won't tell you not to. On the other hand.....all tangs, like the eel above, are...well, they're heavy poopers. Water quality may start to suffer, especially when you factor in the next fish on the list......
The Blue Hepatus Tang, or Dori, as you put it. I know how pretty Blue Tangs are. I also know how fragile they can be when young. How easily they succumb to parasites, and how quickly they can be affected by stress factors such as space issues. I also know how BIG Blue tangs can really get. This is a guy who, if he survives the young phase of his life, will need a bigger tank to live some semblance of a normal life span. I'm sorry, it's just true. Blue tangs have been popularized by the movies and by their breathtaking blue color, but they were NEVER a fish that should be put in anything less than a 120 g, 6 foot long, tank. 180gallons would be better.
Domino Damnsel: As much as I hate these fish, I have to admit I also love these little guys. You will never meet a more pugnacious, irascible little squirt in the ocean as you will with the damnsel family. There's a very good reason I spell my fish the way I do, and you will find that most of us have some sort of love/hate relationship with these buggers. Some of us (Meowzer included) have more of a hate/hate relationship with them. They are feisty, they are extremely territorial, and they will go fin-to-fin with fish 10 times their size to protect their territory. That being said, there's nothing wrong with having them in a 55g tank as long as the tank mates can hold their own. Problem is, a new Heniochus butterfly would be doomed from the start with a Domino in the tank. Ya gotta respect the tenacity of these little devils.
Picasso Trigger. HOOOO Boy. Here's another one that I personally love, but has no place in a 55g tank. Triggers (and Hummas in particular) can have serious mood swings, especially in a crowded tank. That cute little puppy dog you brought home to add to your tank can very easily become the untrained pit bull of the tank as it grows. And again, like the tangs and the Eel...big fish, big waste producer. Remember that as water quality goes down, stress factors on the fish go up. Imagine living in an unwashed bathroom at the Turner Field stadium for months on end....maybe you can manage to stay germ-free, but even the simple SMELL is gonna get to you eventually. Poor water quality has often been linked to increase in disease and early fish mortality. Again, even though many sites reference 70 gallons for this fish...I'd say a 120 to be really healthy.
Clownfish and Yellow headed sleeper goby: I'm lumping these together because it's after 5 and I'm still at work. I want to go home now. ;-) Both of these fish should ordinarily be fine in a 55g tank, but all the extra bodies in the tank is probably putting the clown (who, if she's alone, is almost certainly female) on edge and making her crankier than even most female clowns usually get. You said a "nemo" clown, which I'm hoping means an Ocellaris or Percula clown. And just remember -- female clownfish are often as devilish and territorial as the rest of the Damnselfish family to which they're so closely related.
What does all this mean? Well, it's your choice. They're your pets. You wouldn't throw a half a dozen dogs in one small kennel, would you? Fish may not be as "evolved" as our furry or feathered pets, but they still have similar chemical responses to their environments....and we still have a responsibility to keep them as healthy and "happy" as we possibly can. If I was in your shoes, I would consider two options: Either sit down and decide what fish I really want to see in my tank at the end of the day and consider giving some back to the pet store, or else start searching my local craigslist for a larger tank.
Heck, nevermind. Like I said when I started out. My vote is always going to be for a larger tank. 120g minimum. 180g for a really good time. And once again, I'm so sorry for the loss of your Bannerfish.
Good luck!
