fish dying in my reef tank

Maybe someone can help me. I have a 90 gallon tank set up with live
rock(from tampa bay sal****er). The tank is about 3 months old and has
been doing very well. All of the levels check out ok and water changes
are done every 3-4 weeks at 20-30 percent. Recently, I started buying
fish, and with the exception of a few most of them have died.

I have the following items in the tank:

coral banded shrimp
fire shrimp
5 peppermint shrimp
2 small manits shrimp
There are many urchins and there are 3 anenome's(1 condy, and 2 bulb
anenomes)
2 tomato clowns
1 scooter blenny

The follwing fish have died:

Yellow tang
Naso tang
bicolor blenny
black blenny
mandrin
cardnial fish

My question is this....What could be killing these fish....Could it be
the urchins, anenomes, or maybe the shrimp. I am baffled because the
rest of the tank looks completely healty and everone else is thriving.

Thanks,

Marshall