Clown Fish

To all,
I would like to know if someone could point me in the right direction
regarding Clown Fish. I have now bought 2 sets of 3 Clown's. I put
them in my 55 gallon tank that is mostly live rock with a couple of
other fish. The tank has been set up for about 6 months now and have
not had any problems with the other fish in there, I have a
decorator, emerald and boxer crabs. All are doing well however, I add
the Clowns and they seem to do grate for about 3 weeks or so then all
of a sudden they will stop eating and then die off one after the
other. All other fish are still doing grate and I can not seem to
seen anything wrong except they stop eating. Would really like if
someone could help out with any info
Thanks
Wilbur