I had an octo in a 20 gallon tank when I was around 12. It came in as a pregnant female, she lasted from August to Halloween, she laid her eggs probably early October, they hatched over several days and were in the tank for a day or two before they disappeared. My mom contacted the New England Aquarium and they suggested newly hatched brine shrimp for raising babies.
Overall she was a model citizen, ate all the crabs that I had collected over the summer. She did tussle with the spiny boxfish I had in the tank, so that went back to the LFS. She could have easily escaped...I was using a HOB filter with the standard Pen Plax lid....it was 1982. I got lucky that she never escaped, maybe inked once and spent most her time in a conch shell.