Some very good advice in this thread. The two cents that I would add would be that while 80 gallons might seem like a big number, for the fish you had, it's actually not that big of a tank. I'm not sure of your tank's exact dimensions, but if it's in the range of the 72 gallon bowfronts it's probably about 4 feet long. I'm not sure what size they were when you got them, but the bicoloured angel's adult size is about 6", the yellow tang can reach 8", and I'm not sure of the exact species of butterfly fish that you had, but I'm going to assume based on the common name that it's what's also known as the Latticed Butterfly (Chaetodon rafflesi), which has an adult size of 6". If they were all adults and lined up head to tail, they would be almost half the length of your tank. It very well could work long term, but I would consider that to be a pretty well stocked 80 gallon tank, plus you had/have clowns in there.
So, question 1 - how far apart did you add the fish, and what was your schedule for testing ammonia as you stocked the tank? It sounds like you tested it at least once, but while adding one more fish to an established community likely will have little affect on your ammonia levels, doubling or tripling the number of fish in a system (from one to two, for example) definitely can. Also, a dying nem in an 80 gallon can put off a whole lot of ammonia, enough to have possibly overwhelmed your yellow tang. Did you test for ammonia the day the yellow tang died? It's hard to tell what sort of a timeline this all happened on from your post, how many days between fish losses were there?
Assuming ammonia/water chemistry wasn't the problem (it may or may not have contributed), something else has been killing your fish. While I'm sure there are cases where fish have strokes or heart attacks and mysteriously die for not visible reason, in aquariums you can usually discount those altogether. That leaves us with three other options: Aggression, disease, or malnutrition. Chronic stress could be in there too, but really it just serves to exacerbate the three acute causes of death. This leads me to question 2 - what sort of quarantine procedure did you use for the fish you added? If the answer is none, There is a very good chance disease has played a role. I know you've been keeping fish for a while, but the really nasty killers in the marine world are not always as obvious to the naked eye as some of the fresh water diseases, especially on light coloured fish. The two most obvious things to look for on your surviving fish would be marine ich (Cryptocaryon irritans) and marine velvet (Amyloodinium ocellatum). Marine ich, AKA white spot disease, is usually pretty obvious to the naked eye once it reaches a lethal level, but unless you were looking closely or knew what to look for it's still possible to miss. In it's worst form, you should even see little white spots on the fishes eyes. Marine velvet however, can be subtle enough that unless you knew what to look for you could miss it, and it can take a fish from 'normal' to 'really quite dead' in less than 24 hours. If the fish were not quarantined and prophylactically treated, the chances that you don't have at least some marine ich present in your system is pretty low, which doesn't always mean it will kill them, but it does mean you have a lower margin for error in the level of stress your fish can handle before it can overwhelm them. It's a very common misconception that you can speed up a cycle by using someone else's 'aged' water, as the nitrification process happens in the substrate, not the water column. What there is plenty of in the water column is fish pathogens, and fish stores are especially notorious for being breeding grounds of disease. Most experienced hobbyists go through great lengths to keep water from fish stores out of their systems for that very reason.
If disease wasn't an issue (though I strongly believe it played a role), you have to look at aggression and nutrition. You would have noticed the aggression if it was present, but you'd need to watch very carefully to know if nutrition played a role. Butterfly fish are notoriously difficult to get to eat in captivity, and many die from complications related to starvation. By the time you see them in a store, most fish have spent at least a couple of weeks in transit, and many of them have barely eaten, and were not being given the kind of care they need to train them on new foods. Most LFS's (even the good ones) are pretty bad at making sure each fish in every tank is getting enough food. When I first started out, the only powder blue tangs, butterfly fishes, and anthias's I had ever seen were the ones in store tanks, so I actually had no idea that what I thought was 'normal' was actually how those fish looked when emaciated. Now that I have a healthy Copper Band Butterfly and can compare what they're supposed to look like with how they look in most stores... it's actually quite heart breaking. Chances are good your butterfly wasn't eating enough and that it was under-weight and had little defence against disease or any issues with water chemistry.
My advice - If your tank is having trouble keeping fish alive, you don't even need to be thinking about things like whether your phosphate is too high to keep corals. Find out what is killing your fish first, and address that problem or problems. If your system can't sustain fish, it's absolutely not going to sustain corals, and taking drastic measures to reduce phosphate is only complicating what you should be focusing on. ID any diseases that might be present, and take the established steps to eliminate it as best you can. Adopt a QT strategy that works for you. Research your fish choices and be prepared to go the extra mile (usually in a QT tank) to get finicky eaters fully acclimated and gaining weight before introducing them to food competitors. Stop taking drastic measures to eliminate this or that perceived problem as soon as you notice it - slow and subtle is the name of the game. Save for marine velvet, you'll usually kill more things trying to 'fix' the problem than most 'problems' ever kill.
ANd if you read that whole post, you've got more patience than I usually do.
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