My own personal rule is greater then 4 times the fishes length from side to side and 1 and 1/2 times the fishes length from front to back. That's just me so 16 inch O imo would need a tank longer then 64 inches and deeper (front to back) then 24 inches or at least those deminsions. I just feel fish need room to get a bit of exercise. That's just me though I have a friend who has kept a single O in a 55 for a few years and it looks healthy and happy as can be.
