First off, what exactly is your current stocking list? Because i was looking at your "My Tanks" link and if all of those fish are in your tank right now.. well then i'm pretty sure i already know what the problem is. Half of those fish listed need tanks over 120G and you have them all crammed in to that 75G, which is not good. And you have 20 fish in a 75G tank. lol.
Also, can you give us the tank parameters?
Are the main ones, but since you do have coral, give us any other readings you can test for, Phosphate, Calcium, Magnesium, Alkalinity.. and any others you can test.
Also, please give us the exact readings that show up, and don't just tell us that "the levels are all OK" Because your definition of OK might be our definition of.. death to all the fish in that tank.