yellow tang sudden death?

Im fairly new to the whole saltwater I have had a 29G bio cube but i just upgraded to a 46g bow front with a 20g sump, to find out now that i didnt upgrade enough. I have right now in my 46 two platnium clowns (a pair) and one snowflake clown, with a dragon goby. I have a serpent star, snails and hermits as well, for coral i have a kenya tree some frogspawn and two anenomes, and some fuzzy mushrooms. I went to my LFS two days ago and was looking at yellow tangs (small) and he had two off them. They were kept alone by them selfs without any lights over them besides just your normal room lights and sunlight comeing threw the glass. He said that hes had them for about three weeks now and they looked healthy, they were eating i saw him feed them. The next day i decided to take some of my water up to him and have him test it.  Everything was good my alk was a little low but other than that everything was okay, i didnt see what the exact numbers were though. SO i bought the yellow tang took him home, maybe a 25min drive and acclimated him for about 40min and then put him in. He seemed fine and was swimming around, until i came down this morning and found my serpent star and hermit crabs had almost eaten his whole body already on the sand bed. I was wondering if anybody has any advice or info of why this happened. I dont know if i acclimated him wrong or if it was because he wasnt under any lighting until i put him in my tank, oh i have two t5s and one AI sol. Or if there could be something in my tank that could of killed him. Any info would be great thanks.

the lights were on when i put him in, and i poured in in a container and had a drip line that poured into the container until it was full, took about 35 min.

Thats good to know. Is there any way to see that the fish is stressed out.

thanks dan9554880 appreciate you to take the time to answer some of my questions. I will take more time and do some more research before i buy another fish. Thanks.

I agree with the post above but u can keep a yellow tang in a 46 if the fish is small but it won't be able to live there forever because of the tank size. U can also look into other types of fish such as drawf angel wich will be able to live in that thank size

I would not spend overly long on worrying about this- fish can survive with little wear and tear being dipped into very low salinity for a fairly long period of time so its doubtful your acclimation has much to be improved upon.  I typically feed when I add fish and try to re-arrange things to avoid too much territory issues.  Could be your fish ganged up on the tang, but without you having witnessed it who knows.  

ya all the fish are on this table(stand) and there are little boxes for the fish to be in and the water flows through the whole thing. And he said that he had the tangs for three weeks in there. There is no lights over them only natural sunlight and normal room lighting.

Yellow tangs should also be the last fish you add to your tank. They are highly aggressive and territorial

Thanks everyone, I went back to my LFS and told him what happened  and they gave me another yellow tang(small) for almost nothing. I tried to do as much research as i could and i tried a couple things diffrently with putting this tang in my tank, he seems to be doing fine and he is swimming around and eating like a pig. I think the biggest things that made a diffrence was i kept the bag with the fish floating in my sump the whole time while i acclimated him. and then i kept my lights off the rest of the day and night, then slowly turning my lights back on the following day. Im sure alot of you are reading this thinking what the heck was he doing the time before, but like i said im a newbie. Thanks to everyone on this forum im sure i wont kill as many fish, and be able to keep more money in my pocket (wifes pocket) Thanks again