A. aculeatus doesn't normally eat fish, but with my last male octopus, I experimented with some of my SW molly fry, just to see...sure enough, he stalked and ate them. These were very small fry - maybe .25-.5 inches? Certainly not enough to sustain an octopus long-term, and I only did it once or twice as an experiment (and with close supervision to make sure they molly fry did not harass him).