Octopus cyanea is a fairly large, day active octopus (that's why it's also called the Day Octopus) - body to 16 cm and arms of 80 cm - You'd need a big tank, maybe a 90 or 100 gallon.
Very common on coral reefs in the ropical Indian and Pacific Oeans - it excavates lairs within coral reef and rubble.
It eats crabs, other crustaceans and fish.
Can produce large clouds of dark brown ink.
Has a courtship ritual where the male holds up its third right arm and waves it at the female.
Eggs are small and hatch into planktonic juveniles.
(This is a brief summary of information from Mark Norman's Cephalopods, A World Guide)
Nancy