What killed my triggerfish?

Hi

It is a sad day for me and my family since we lost our very active and
lively picasso trigger.
The fish had been kept alone in a 55 GA tank with a wet dry filter.
The fish was very cool with him allowing us to pet him for long period
of time, he was very lively too lively even sometimes as we were
watching tv he wouldn't stop moving rapidly around the tank begging
for food and getting our attention away from the tv.
Two days ago he slowed down being barely interested in food, then
later on that day it went hiding into a piece of coral and beside for
it's gills and eyes, wouldn't move.
Last night as i came from work, it was still there, i decided the
following morning i would do a big water change suspecting ammonia or
nitrite poisoning as the fish didn't show anything wrong on its
skin.Only its behavior had changed.
This morning as i was ready to start changing the water, it came out
of the coral and laid flat on the gravel still breathing normally.
I decided to scoop him out and placed him in my cycled quarantine
tank.It seemed to recover for a little while but suddenly died.
I tested the water for ammonia and nothing showed, i also tested for
the dangerous nitrite and there were signs of it but very low.I tested
my other healthy tanks and they all show the same level of nitrite but
all the fish are fine and don't show any signs of stress.
The trigger seemed to have become paralyzed progressively and died in
full color.
His tank was a mirror back placed close to a large window and needed
often scrubbing as algea would plague it at times.
Another thing that might be of interest was the fish wouldn't eat
anything but "That fish place" cheap sal****er flakes. I have given
him other better brands pellets, whole mussels, clams, squid and
pieces of shrimps to no avail, he would play with them but not really
eat waiting for it's flakes.
When the flakes would finnaly come, he would go nuts for them. That i
think started preventing my fish from getting a lot of the nutritient
he needed. His stomach became slightly hollowed and i could never
change his feeding habit.
His growth seemed also stunted as it never really grew a lot.
Could have he died of malnutrition?
I am just trying to give the expert the most info i can as i would
love to know what went wrong.
And as far as me and wife can recall, nobody remembers placing an arm
that had touched perfume, bleach or soap in the previous days.

Thanks for your time and take care of your fish

Patrick
(Answer here as the email is bogus)