Hi Nick,
I was just looking at the descripton of Octopus cyanea (Day Octopus).
Like its name, it's day active. In some color patterns it has black false-eye spot (no iridescent ring) and brown arm tips with regular rows ofwhite spots. Does your octo match this so far? If you have this species, you have a younger one because it's mantle can grow larger than 6 inches and its arms over 30 inches.
You need to remove any dead animals from your water. Clams, scallops, etc. seem to be especially bad at polluting water. The only exception is dead food for your octo, such as thawed frozen shrimp, but if he doesn't eat it within several hours, you should remove that, too.
Your octo likes crabs and other crustaceans for dinner, also some fish.
Nancy