My first suggestion whould be to stopping adding fish in large numbers. If Your fish are dying why replace with 4 more, then 6? One at a time whould be a cheaper and safer test method.
Isopods are usually something youll notice on the fish.
Ther not allways that big but if your looking for them you should find them.
Who are you getting your fish from? safari or hillside i doubt youd get that many bad fish from. Creatures on the other hand you could probably buy by the dozen and they all die off.
My best advice is never put anything into your reef that hasnt been in a seprate quaretine tank for at least 1 month, That way you no if its the fish or something in your tank. If it lives through QT for a month and come's out healthy, Ya put in your reef and 2 day's later its dead. You know there something wrong. The way your doing it now its hard to tell if its your tank or a pre-exsisting condition the fish had.
Any other symptoms whould help in figuring this out though, Were they gasping, flashing, showing any odd coloraion. anything?