I know copper is bad for octos, and I remember reading that some other metals are bad too - does anyone have resources on this? I remember thinking stainless steel is the only metal that is safe since it won't rust - is this true? Is ANY rust of any metal going to be harmful or toxic to octopus?
I don't know for sure about this, my chemistry knowledge is pretty weak. Generally speaking however, its best to have as little metal as possible in your aquarium setup, especially metal that is in contact with the water.
I assume that most metal unless protected with a special coating will rust eventually due to the nature of salt water. I know some metals are resistant because the screws in my protein skimmer have yet to show signs of deterioration and they've been in my system for over a year now.
Do you have anything specific you are worried about? I may be able to help suggest some non-metal alternatives.