Wanted to say at the outset that an octopus is an incredible lifeform and is a lot of work to keep. But it is a highly intelligent and fascinating animal that definitely deserves it's own tank and specialized care.
I've kept an octopus twice, each one for about 6-9 months. Both were caught on a rocky shore in southern Japan. Sorry, I don't know the exact species as there several dozen species in the local waters, but definitely distant relations to the blue ringed octopus.
I never kept one through the winter and both times they were small when I caught them in the spring ... and rather large (although it's hard to judge) when they went back in the ocean in the fall.
Ok, getting to my point. In both cases,
fish were definitely food. I would net some non-ornamental fish once a week and this was food for the octopus.
Dumping in a 5 gallon bucket with about a dozen small to medium fish resulted in all the fish being rapidly caught. I don't recall any time when it took the octopus more than 10 minutes to capture
all the fish. The point being that an octopus has the capacity to
capture and hold multiple fish all at the same time (and while all fish are bucking and struggling violently to get away).
I never saw either of my octopus let a fish go after catching it.. the evidence that there was ever any fish in the tank simply disappears.
This is the reason for my post. If
PherFox could see "dead fish" in the tank, then it was likely not due to predation by the octopus.
I doubt the behavior of an octopus varies all that much across the genera. I'm certainly no expert, but from the behavior I witnessed, it was pretty clear that they just don't drop fish they have caught. They are more like an 8 armed pig in an apple tree.