I want to learn sal****er techniques

I have a 90 gallon sal****er tank for about three years now. I have four
fish, a lionfish, a black trigger, a sailfin tang and a 'comet'. These are
healthy and have grown some since i've had them, and all have good
appetites. I have ceased checking water condition(nitrites, ammonia) because
I have not ever had any readings which were out of the normal range. In
fact, every time I tested, all negative contaminants were very low, barely
registering, in fact. So, i haven;'t been checking anything except temp and
salinity. I seem to be able to keep this tank and it's four residents
healthy.
my question is, what kind of things must i do if i want to keep some of
the more delicate fishes? I tried to keep butterfly fishes about two years
ago, and I bought about three, and they all died, sooner rather than later.
I suspected I was doing something wrong, making conditions too stressful,
and that killed them, but i am not sure. I use charcoal in the filter, I
run two filters, with no ultraviolet sterilizer, and no protein skimmer. I
change the filters every few months, one at a time, alternately. I also do
water changes about once a month, usually about ten gallons, and i use
distilled water. I mix the salt right in a container in the tank, hanging a
32 ounce pyrex cup inside the tank, with two cups of salt , and i stir
distilled water into the cup until the salt dissolves. I mix it to the
strength recommended on the bag(1/2 cup per gallon). I don't know what else
to mention, except that i feed them frozen brine, frozen mysis, flakes, and
some other frozen foods. I have no live rock, and I have standard
fluorescent aquarium lighting. I am hoping to get a new tank soon,
hopefully it will be in the two hundred gallon range, and I ould like to
have a few more fishes than these four. if anyone can give me suggestions
for keeping sal****er fishes healthy, or at least tell me what practices
are truly important, since i have read a few books and am familiar with most
of the common practices, at least theoretically. Should i get a sump-tank
type filter, or some other special filtration? Is a prtein skimmer going to
make a stron positive difference? Like i said, any advice or instructions,
or information on what to read, would be appreciated. I don't want to go
back to fresh water, at least not unless i still keep my sal****er tank.
Thanks, Mike.