whatever your lfs guy says, take it with a grain of salt...
almost everyone disagrees on your current set ups...
a 120 gallon "shark tank..." see above
no matter how tiny, a ten gallon is NOT suitable for any sort of tang... at my lfs we just got in some 3/4" yellows, and theyre in a 75 gallon and they swim all day along the front of the glass...
same with your "10 gallon eel tank." not possible.
listen to the good folks on these bb's
Drew