Another fatality...

How old is the tank? Do you hear any "clicking" coming from the tank at night/ lights out? Could be a Mantis.

No, no clicking. But then it would be hard to hear because of the water trickle going over the overflow. But, I'll turn off the pump tonight for a while and check it out. Thanks for the idea. I have had the tank for 3 months.

Look at it at night with a red lense flashlight. A mantis may be able to detect this spectrum, but it isn't as bright/obvious as a regular lense. Sit still in front of the tank and wait. Good luck.

Should be able to pick one up at an Army Navy store or a good camping gear place. Maybe a good hardware store. It's a reg flashlight and as accessories they have red and yellow len's you can slip in =)

I'd suspect your serpant star. I do not think they are reef safe...just my experience. I had one that was about the size of yours and it killed my jawfish. I saw it in the act so I know it was the serpant. The gobie is a bottom dwelling fish right?

Then again it might be other hermit crabs, they are visious little opportunistic feeders, killing snails for the shell, same with other hermits. Also able to ambush little fish that sleep in the rocks.
It could be a serpent star tho the green brittles are the most commonly indicted as fish killers. most of the others are fairly passive, tho large brittles should be target fed a couple times a week, just scavenging may not be enough to keep them going without preying on desirable specimens.
Mag instruments makes an accessory pac for thei Mag Light flashlights that includes the interchangable colored lenses. Some of the GI style flashlights (angle head) also have the colored filters as well. If you can find red cellophane you can make red lense cover in a pinch

I've done the red celophane too. Would other colors work as well?