Hey man I am in the throws of starting a 30tall SPS,LPS,zoa tank.I say that 30g and under is a nano and 5g and under is a pico.
You cannot keep a single bngai cardnal in a tank,they are a schooling fish and if you keep one it will be very stressed out and die.
What kind of clowns are you looking at?false percs?true percs?ocellaris?
you will hafto choose between a school of bangais or 2 clowns.Otherwise it will be too much of a bioload.
Either the firefish or the gramma should be fine.
How often do you do your waterchanges?
Oh and become a member of:
http://www.nano-reef.com I am a member and its alot of fun!Just say FUGU!!! sent you