Well, at least you were set up to fail after they got the tank cycled...
If this is a 20 gallon tank, then the lbs per gallon rule of thumb for sand and rock is right where it needs to be.
The sad part is that they let you go home with fish that need much larger tanks than a standard 20 gallon. A yellow tang needs a 6ft long tank, like a 125 gallon, to thrive as they are one of the several open water swimmers that we find in this hobby.
The same is most likely true about the angel fish and the close quarters either led
to aggression from the two large fish or adding them both at once into such a small tank overwhelmed the bacteria base you built up when you cycled and they died of ammonia poisoning...either is likely.
As for the mandarin...awful rate of survival in the home aquarium. I've tried and it is an expensive venture. These guys eat pods
...for the most part only pods
. One in my 55 ate thousands of them every month. A 20 that isn't established doesn't have a shot at feeding one. If the ammonia poisoning didn't get it, the starvation would have.
Then there is the anemone. A photosynthetic creature that needs intense lighting to survive. I highly doubt you were supplied with such lighting so I would suggest returning it before it bleaches out and dies, creating a nasty water problem in your tank. Don't worry, the clowns don't need it to survive.
In better news, two clowns is great for a 20 gallon. I think every tank needs a clown pair.
In reflection, the key in this hobby is to go slow. Adding so many fish to the tank without researching them is causing you to go down a path of continued issues and failure. Very frustrating for anyone who wants to pick up this hobby! I suggest checking out liveaquaria.com. They have GREAT recommendations in terms of what fish need to live healthy lives in our closed systems. If it doesn't fit your setup, then I would return it quickly for either cash or store credit until they get something that will match your system.