We had removed a large Bristle worm about 2 months ago
 

but we just saw another one today. looks to be around finger width and 5" long, so was not as large as the firsy one, but is still pretty big. Would that have been a cause and should I try to remove it?

Wait I'm not sure I understand... how many dead fish are we talking about here? 2 dead tangs in quick succession?

Yes if there's a dead fish in there you should absolutely remove it. Also test your water for ammonia, nitrite and nitrates. You don't want to start a chain reaction and crash your whole tank.

The question is why are your fish dying - how long have you had the ones that died? when was the last time you added a new fish? Are you sure they're getting an appropriate diet? Is there aggression issues in your tank that's preventing some fish from being able to eat or stressing them out badly? Are there any symptoms of disease on your other fish - specifically look for small white spots on your tangs, as they're most likely to show visible symptoms of ich if this is an issue.

If you experience multiple fish deaths close together, there's likely a much bigger problem going on. Either disease, water quality, nutrition or the social structure of your tank is out of whack. What kind of tangs are we talking about here? They're not all created equal, and if you've got something like a sohal in with 3 smaller and more docile tangs in a 150 gallon aquarium it's basically a recipe for carnage.

What asylumdown said but:

Both a powder blue tang and an A.nigricans (looks like a powder brown tang but has no orange on it) will die for no apparent reason most often taking a few of their tankmates with them.

Guys, he's talking about finding a second bristleworm :) And no, the worm didn't cause anything, they're good to have.

So even though the size of the worm is about 5", he is fine to be left alone?

Ok, so this is what occurred. I had 16 fish. 4 Blue green chromises. several damsels and a percula clown. A Flame angel. A goby. One yellow, Tomini, Sohal and Naso tang that were all about 4" in length. Last night when I arrived back home, I found the Tomini, Naso and Sohal tangs and the Flame Angel all dead and trapped in the live rock and all in the same spot.

It happened Monday as ALL the other fish looked very stressed and sickly. At Wed, they were all looking better, and i found and discarded the dead ones last night. They did not appear attacked though hard to tell, but very decayed. I did another water change, and all 12 remaining fish, crabs and corals look completely fine.

Any thoughts as to what may have happened to the 4 fish that all died in the same spot? under the live rock?

Probably one fish died caused a chain of reactions. The other three died and the current pushed them to the same spot?

Location of the fish dying probably has nothing to do with it.

But what is your plumbing/connectors and such made out of? Do you have any copper fittings in the tank? Any steel that has rusted such as hose clamps etc?

Can you take and post a full range of perameters? Nitrates, phosphates, temp, PH, etc etc.

What do you feed? How often do you feed? Do you suppliment the tangs with any nori or anything else?

Like Adam said, tangs can be ich magnets, and the fact that there were that many tangs in a 150 could have been an issue (stress). Any sign of ich on any of the other tank mates?

They drank the Kool-aid.