Disappearing Fish!

[SaltyDog08]

So I've had my tank set up for close to a year now. Well I had a 40 breeder then upgraded to a 75 but with all perfect water and the same rock. I haven't really been buying a lot of fish because I'm working on other things right now. But anyway I've had this one clown for probably 5 months now and I had posted a tread about it hosting xenia and everything was going awesome then it just disappeared. I got another clown a week later and it was doing great then the next morning it was gone! I'm not sure why just the clowns are disappearing. I also have two firefish both 2"-2.5", a yasha haze shrimp goby 2", and a candycane pistol shrimp 1.5". I know clowns aren't natorious for darting so I don't think it would dart in the shrimp hole. Any suggestions to what may have happend and how would i fix it?

 

[lizardarm]

Is it a reef ready aquarium? (built in overflow box)
I had a clown years back that had somehow managed to jump into the overflow box. Lived in there for several months before I finally got him out.
Or, you might have a predatory crab in there somewhere. Do you ever put a flashlight to your tank after the lights have been out a while?
Just the first two things that popped into my head.
-Chris