Welcome to TONMO, the premier cephalopod interest community, and birthplace of #WorldOctopusDay and #CephalopodAwarenessDays. Founded in 2000, we are a large community of experts, hobbyists and enthusiasts, some of whom come together when we host our biennial conference. To join in on the fun, sign up. You can also become a Supporter for just $50/year to remove all ads and enjoy other perks. Follow us on Twitter for more cephy goodness.
Silver sides will definitely the better option as it has the marine fatty acids or something like that which freshwater feeder fish lack. Freshwater fish might sometimes have copper in them from copper based medications, This can be extremely fatal to cephs.
There are lots of different foods you can try with your octo. Their first favorite is always live crabs, but you can try pieces of fresh scallop or marine fish - also frozen shrimp (thawed and same temperature as the water).
How big is your octopus and do you know what species it is?