Humans can live in 6x8 prison cells for 30 years. You can live for days on nothing but urine.
Remember that in the wild, these animals have reasonably-sized territories. I feel strongly that as a marine aquarist that is buying an animal that was forcibly removed from its habitat, that I strive to provide it with the highest-quality home I reasonably can. For a peacock, that means at least 25-30 gallons, a dark tube, lots of rock and rock rubble, some live foods, a good, varied diet, and somewhat subdued lighting; not to mention, heavy filtration / skimming.
IMNSHO, keeping a peacock in a 12 is cruel. YMMV.
Dan