So you've lost 2 maroon clownfish, a horseshoe crab, a powder blue tang as well as a niger trigger? Something is very off. Have you noticed your water is murky? Spotted any bones? You may have a predator in there, whenever I lose a fish I at least find some part of it a while after.
You have a serious problem. They must have died in the tank (or been murdered). I had a gorilla crab that came in some life rock and took out a couple of fish before I realized what it was. I'd tear everything apart and 1. Look for bones. 2. Look for a hitchhiker that might be causing you problems.
Definitely stop adding anything new until you have this figured out. If I recall from earlier in the thread you've had this tank up since October. That's 5 months max. A reef tank will take longer than that to fully stabilize. You may be adding too many fish in a short period of time. Water may be off and the new fish can't take it. They die usually hidden in the rocks. Within 24hrs they can be eaten up by all the critters in there no problem.
How can I found out what's in my tank I just cleaned it yesterday removing a lot of rocks I had into a bucket so I can see what's going on but how would I be able to fish out the problem if it's a hitchhiker???
There can be all kinds of things. The for rolls crabs will hide themselves really well, especially when the rock is out of the water.
Moot point anyway because I doubt a gorilla crab could take down a 6" tang.
I can tell you that tangs love finding a thin space in the rock or most often between the substrate and a rock. You'll never see them until you lift the rock. Maybe that's where it went then your crew took care of it. That happened to me once. I couldn't see it and my nitrites spiked 2 days later. On 3rd day I had to tear apart the tank and found the body of the tang wedged under the rock, slowly decomposing. Crabs and snails couldn't get to it. Can you remove the rock for a while and leave the fish by themselves so you observe for a while?