I'm thinking it's a mantis or pistol shrimp. Do you hear any "clicking" at night particularly when the lights first go out? I had a mantis in a 33g tank for 8 months before I found it. There was a 3" crab in a friend's 180g tank for over a year before he found it. So, sometimes things hide VERY well for a LONG time!!

Used to have the same problem, until I came home one day and noticed a Mantis shrimp scrurrying across the sand. He had been there for years and I had no clue, every once in a while I would hear a click but attributed it to a snail falling off the glass. Took me a long time to find his lair.

Thanks for the replies . I have set a trap and will see what happens.

Never had any luck with a trap, I had to pull out each and every rock and inspect it to find it's "little nest". And as luck would have it, it was literally the last rock I pulled out in the tank where he was hiding.

Hope you have better luck<g>

I am thinking that perhaps the fact that you have put a large bio-load in what is basically a brand new set up may be your issue. What brand and how old are your test kits? Have you taken a sample of your water to your LFS to confirm your readings? If your problem started more than a month ago that would mean your tank was only two months ish old, which is fairly new to be adding that many fish at once. It is possible that your fish are dying from water quality and your clean up crew are taking care of the carcass.

I would echo what many others are saying about a crab or mantis shrimp as I had the same issue when setting up my tank a few months ago.

I had about 70lbs of liverock in a 90 gal tank and 5 chromis. All fish seemed healthy but every couple of nights a fish would disappear by morning. I could never find any of their bodies.

One morning I checked the tank and found a dead Gorrilla crab. I figured that it was the cause of all my problems but I kept losing fish after that. I ended up finding another 3 pretty large crabs after that.

So as others have said you could have alot of crabs in the amount of liverock you have as I had at least 4 with only 70lbs of liverock.

I didnt use a trap for the crabs but i did find that when i fed frozen brine shrimp, i could see crab claws coming out of the rock trying to get the food. I then was able to pull the live rock out and use tweezers and a screwdriver to push the crab out.

Just want to confirm a few things.
1. Have you lost any tangs?
2. When you say refugium do you mean Quarantine Tank (QT?) which is a tank completely separate from the main tank.
3. Using a flashlight do you do you see any salt like spots or any whiteish coating on any of the fish?
4. Are you adding any additives to the tank?
Thank you