Hi Mike,
I'll echo what Drew said....IMHO, a 75 is the least size tank I'd consider for any Tang, and even then, only the smaller varieties. Some have kept them successfully in smaller tanks but it's tough on these fast, free swimming fish...many get stressed and have Ich problems in smaller tanks.
I have a 55 with these fish: 2 Ocellaris Clowns; 2 Neon Gobies (Gobiosoma oceanops); 1 Pygmy Cherub Angel (
Centropyge argi); and a Lawnmower Blenny (
Salarias fasciatus). I consider this a pretty full load for my 55 reef.
You've indicated before that your 55 is Fish Only for now and that you are planning a large tank. I'd recommend passing on the Tang for now and look for fish in the 1-3~" range, adult size. A 55 isn't really that big a tank for marine fish.
Robert Fenner's
The Conscientious Marine Aquarist has a pretty good rundown on fish. Scott Michael's
Reef Fishes is another good reference for selection of fish.
BTW, IMHO, those 2 Damsels of yours are going to treat your tank as "their" territory and may harass anything that goes in next.
Dick
