I just went and looked at your other posts. Also I went to the original thread of this gumball tank here is the quote I pulled.
*I was gonna put my pigmy octo in there...he eats outa my hand now...in my 55 tank.
But i'm gonna have to give him a hermit or snail every couple of days....or a small crab.
He might be too messy for that size tank tho.*
That was the creater Gololf comments. I am not too certain if the exchange ever took place but if so prolly did not last all too long. You would have to do continuous h2o changes.
That being said, he are some basic reasons:
Octos can and will crawl/hide/live under an undergravel filter.
Undergravel filters in most cases do not do enough to filter an octo tank, beside giving them a place to always hide from you
Octos are very messy. Atleast 3 times as a fish their size
Alot of pygmy octos are misidentified and are actually large juv.
In the event of a power outage watch out for smaller tanks
Taking this into consideration, Colin was suggesting a 30 gal. Octos are basically species only, very few other inhabitants do not become food, or visa versa.
Hope this helps, there are several more reasons if you are still curious but this is a good start.