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Is it possible to keep a large angel in a 75 gal tank? If yes, what would it be?

yes kung mga 3 to 4pcs max and my bet will be majestic angel, blue face angel, queen angel and blue koran

Is there an angel with adult size below 12"?

There may be a possibility of aggression, especially if you already have other large sized fishes such as tangs or other surgeonfishes/rabbitfishes (like what aquareef posted).

The minimum recommended tank size for one large angel is around +/-125 gallon (as per RC and liveaquaria). But of course, nasa iyo pa rin yun, at depende na rin sa rock arrangement nga tank mo and the number of inhabitants in your existing tank. Bioload lang ang kalaban mo.

Adult Lamarck, Xanthurus, Bluespotted, Bellus, Watanabei and Spotbreast Angels are less than 10" in size.

Yung emperor angelfish kaya possible sa 75gal aquarium? Provided na blue tang , 2 false percula and maybe 5 firefishes lang yung kasama nya?

Sa tang pa lang masikip na bro.

i would not keep any large angels in a tank lesser than 125g.. unless you want to give them away when they grow. I would just go for centropyge angels (dwarf angels).. kahit yung local ones magaganda..