It's hard to hard without seeing the head, but that looks like a Eunicid worm. That is a bad worm, and needs to be removed. They can kill fish by grabbing onto them and quickly pulling back which rams the fish into the rock. They can move at some silly rate like 200 ft per second or some such thing!
I would suggest you remove the rock you saw it go back into and dip the rock in freshwater. If the worm is in that rock he will come out quite quickly when dipped in freshwater. If he doesn't, try a different rock. Don't leave the rock out for too long, and don't leave it in the freshwater for more than a couple minutes. This will probably cause a bit of a cycle in your tank.
Anyway, the worm may be the cause of your fishes demise, but the new tank could be the cause as well. Your tank doesn't have any algae growing yet which is probably why the hermit died. No food = starvation. If i were you I would try to take the hermits and snails back to the store, until there is more for the little guys to eat. There is also no need for 5 hermits and 2 snails in a 20 gallon tank even later on. Too many, they will just starve to death.
It could very well be that the clowns were not in good health when you bought them. Some of them can be quite sensitive and some of them never seem to acclimate very well.
I would also suggest you try to get another small piece of live rock...something with some color (purple/pink is good, green is not so good) in hopes of seeding the tank a bit better. Your rock is awfully white and looks as though there is not a terribly large amount of life on it.
Once you get that worm out, and let your tank sit for another couple weeks monitoring the ammonia and nitrate (no need to test for nitrite in sw aquaria) you can try fish again. I would also suggest you not worry about pH so much, and quit dosing the buffer.
For more information on pH use the link in my signature. The write-up is aimed towards reef tanks, but the part on pH is relevant to FOWLRs too.
Good job keeping a log going! That will be helpful in the future.
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~ Mindy
SPS fanatic.
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