Hello and thank you for your inquiry,
While it can depend on the type of fish that are disappearing on you, the two most common predators that cause this type of problem are mantis shrimp or green brittle starfish.
If you hear a "clicking" noise in your tank there is a chance you have a mantis shrimp in the tank that could be causing the problems. They will hide under and within the rockwork and it can be very hard to see them. There are DIY traps that you can make and set out during the night time to try to catch them.
Green Brittle Starfish are also a commonly sold "pest" that sometimes people don't realize are causing the problem. There are many different types of brittle starfish, but it only seems to be the green colored ones that cause the issue. If you have one of these guys in your tank I would recommend getting rid of it.
Occasionally you will also see what is called a "gorilla" crab come in on rock. Usually they are smaller and won't be able to harm larger fish, but smaller fish can sometimes fall prey to them. Again searching for a DIY trap to catch them can help hopefully remove him from your tank if you have one.
Hopefully that helps with possible solutions to the possible predator in your tank, but if not please shoot us an email at firstname.lastname@example.org for further assistance.
Marine Depot Customer Service