Interesting build Idea
John dory would be cool, can't say I have ever heard of anyone keeping one, how do you propose to catch one????
remember octopus require really good water quality and only live for couple of years max, so would be better to start with octo to get the system fine tuned then once ready go find a small johnny, and if you get sick of it, nice dinner.
btw johndory will most likely require a chiller unit