distraught fish fancier

I have a 92 gallon corner tank. the tank is about 18 months old, and
previous to now I have not lost a fish, although 2 snails died. I have a
sailfin tang, who's fins have sort of melted away, looking like plastic when
it melts from heat. The tang is still active and healthy acting, however. I
also have a green trigger, about 4-5 inches long, very active, very healthy.
I have a yellowtail damsel, also apparently healthy.
A month ago, I bought a marine betta, a marine algae eater, or lawnmower
blenny, a copperband butterfly, and a yellowtail butterfly. After about ten
days, the copperband just went from active, and pretty, to dead overnight,
without a mark on it's body. Then, the lawnmower blenny has gone missing,
not a trace to be found, although, admittedly, I have not tossed the tank's
rockwork looking for him. I believe he is dead though. Now, tonight, I am
find my yellowtail butterfly seeming to be ailing seriously, and I don't
expect him to live. He has stopped eating, and he looks ill. I am distinctly
depressed and discouraged with these developments, and I wonder if anyone
can offer any constructive guidance or information. The other fish ( marine
betta) appear healthy, and they have strong appetites. Did I overcrowd the
tank? Is it possible the trigger harmed any of the fishes, without me ever
once seeing him bother them? Are perhaps the butterfly fishes too
delicate for my type of community tank? Thanks for any insight or advice
you can offer. Mike.