I'm sure everyone here is aware of this, but a lot of lfs's will take back fish that customers bring in. This fish might have outgrown a tank, heck it's owner might have died for all we know.
Not saying lfs's don't overstock because they do, but without the op having talked to the workers there we have no idea what the situation is.
It's nice to see that there are still folks who have faith in their LFS....
To all you who bash them, feel free to go to your "online" supplier and see the conditions in their tanks, just because you can't see in their doors, doesn't mean that this stuff doesn't go on.
Perhaps that fish was ordered for someone with a much larger tank, and it's only in the store for a short time.
Perhaps the store ordered what they thought was a medium fish, and got a XL fish instead, that stuff happens all the time.
Perhaps that fish was brought in by a local hobbyist...
There are many reasons why they could have a huge fish in a 100G tank, just be glad it wasn't in a 20L.
At any rate, if a LFS adhered to the Reef Central Guidelines of what tank should house what fish, these LFS's would have 50,000 Gal of water and be the size of Walmart.