My friend owns a fish store and had a pacific octopus like this one. Octopus-Pacific
It was alot of work. They are extremely messy eaters easily fowling water. 10 g would be to hard to maintain.
My friends regularly opened the lid of the 100 g tank he was in. And the lid had 2, 45# weights on it. Another problem he had was they are nocturnal. Like dead of night nocturnal. He had cameras on the tank to watch it at night when he was home. They are brilliant. He fed it crabs and such in a jar with a lid. Sometimes it would replace the lid after food was removed.