Typically, we don't use air stones in an octopus display tank but because the top must be closed, there must be some form of water movement to exchange air and CO2 in order to keep enough oxygen in the water. This is most easily accomplished with an open sump where there is full air exposure and a skimmer.
It has been observed that large air bubble can be caught in the mantle and may be fatal to the octopus. That being said, others have kept air stones in the display without problems.
It has been observed that large air bubble can be caught in the mantle and may be fatal to the octopus. That being said, others have kept air stones in the display without problems.