It is probably not something you have not checked but ... what is your salinity reading? I noticed Sisty and Dusa acting a bit strangly awhile back and thought to check the salt content. My other tanks are much larger and I have a given "formula" when I do my weekly water changes but I discovered that the smaller tank needed more monitoring for salt when testing showed a SG of 1.021.
I did check salinity it was between 1.025 and 1.026.Unfortunatly Olorin has past, I can't believe it happened that fast.He has been acting a little strange the last few days but nothing I thought to be too concerned about.This morning he was out(no problem there)but his arms were corkscrewed,he wouldn't eat ,and he stayed out all day,I didn't think it would happen that fast.
God bless him he was great!
I know it sounds bad that I lost two octos in one week,Achilles was a hard lesson in octo proofing ,I have to check the seal every day.Olorin on the other hand was old age.At least I don't see any other reason for his death.
I know it is not much help, but if you review Biddle's post he went similarly. Carol mentioned slightly unusual behavior but nothing that shouted warning and then he passed away. It does seem that each animal is unique rather than a species generality as I also remember Greg saying there was a vast range in time and behavior of sesenence. I know what you mean about removing him, I sat and looked at Trapper a long time.
Einy was pretty quick too now that I think about it. I looked at him one night right before bed and he was all corkscrewed and twitching, seemed fine the night before. Dead the next morning. Didn't lose any arms though.