[Fishfirst]

1st off I'd like you to understand that the LFS is there to make money, and thats just what they are doing when they suggest certain fish (especially ones that will outgrow and be chronically stressed in the size tank you have) and most likely, too many fish at once or too many fish too fast. 2ndly but most definately not the cause of your fishes immediate death but still something to point out is "men cannot live on bread alone" in other words, your brine shrimp two times daily is hardly a good diet for any of your fish much less the tang who needs fresh seaweed daily. Pick a flake food, at least two different types of frozen food such as mysis shrimp, krill, cyclopse, emerald entree, clam, squid, ect, and at least one veggie food such as spirulina algae, kelp pellets, or seaweed, and alternate between these.

My guess for your fish dieing is more due to the fact that you added fish too fast or too many at once or you are feeding too much (two cubes of brine shrimp/day can do that) creating ammonia problems and health problems for your fish. Ammonia and nitrite readings should be 0 ALWAYS. So in order to get a better picture of what is going on... I would go through your tank history with us. When was it setup, when did you add the liverock, when did you add your first fish (did you add more than one?), when did you add your second fish ect, when did your first set of fish die, when did you add the second set of fish and in what order and how long appart, when did the second set of fish die, how long did it take you to add the third set of fish after the second set had died?

you get the picture?

a timeline can show crittical errors in fish husbandry.
lastly, if you are changing your filters every month, it can "set the clock back" on your tank... I would NOT remove the filters for months especially if your adding fish at that point in time.

The fish definately sound like theyve been through ammonia poisoning, as well as the invertebrates.