you may want more than 5 lb the usualy amount of live rock for a tank is 2 pounds per gallon or 1 pound per gallon. octopus dont need toys but they need your atention or they will get bord and make a better efort to get out of your tank
Octopuses need both your attention and some toys or objects of interest.
Not all octos show the same interest in toys, and some will keep objects as prized possessions - my bimac did this with the little bottle I gave her. It was a sturdy small glass bottle with a shell and some sand inside, also some water to weigh it down a bit. Insted of playing with it, as I had hoped, she took it right to her den and kept it there. I could peek in and see her sleeping with an arm around the little bottle.