Two years ago we had an experiment with raising briareus hatchlings. Debbie (dbbga) kept a briareus that had eggs. She would take most of the hatchlings, letting them live in the same very large mature tank where the mother lived. She also added mysids to the tank.
Another person took 20 or so, put each one in a small cup with saltwater, a sprig of macoalgae, and always a mysid or two.
I took 6 and put them in a very mature 19-gallon tank, full of small food such as copepods and amphipods and I added mysid shrimp.
Debbie ended up having no survivors, the second person had one live for two months, and I thought I'd lost all of mine. However, after 4 months, my husband was scraping algae off the side and out came a young briareus! I was astounded - I had intended to transfer any survivor to my larger tank after a couple of months. (Little Pod, as she was called, survived only a few weeks longer - I think she was attacked by something in the tank that I never found. ) But this does prove that you can raise an octopus in a mature tank with plenty of food.
I'd also suggest giving away some of the hatchlings - maybe that will raise the changes of having some live.
Nancy