octopus from the documentaires I have watched are excape artists like eels execpt they have arms to work with and have been known to lift off heavy tops covering an aquarium. IMO
they are not good for an aquarium because they will spend 90% of their time hidden away in some crevice within the live rock only to come out at night to feed or to try to excape their enviorment.
As Phamboo mentioned sting rays need a sandy substrate of good depth because they will bury themselves in the sand. They will also need alot of clear sand area to move around. I dont think they would do very well in small tanks or even medium to medium large tanks. Ideally they should be kept in tanks with several square feet of sandy substrate free of live rocks and other obstructions.