I learned from this expereince very early in the game not to overload your tank too quickly.
Now if you had say a 20G tank, definatly 1 at a time, If I could do half a fish at a time I would
but that would be messy.
In my 70, I have added 3 fish at a time MAX. only reason I got 3 that time was a pair of clowns and a blenny I had been waiting for.
LFS will always try to sell you on adding as many as you want. BigAl's is horrible for that. AI told me I could load up on fish that day, but err' on the side of caution, i didnt.