Depends upon which "octopus" skimmer, many of them are designed specifically to ONLY run in the tank, especially those with the Sicce pumps as they'll very easily overheat outside of the tank.
I couldn't say if they are a good skimmer from experience, however it is a good pump, I have them on my ATi bubblemaster, and they certainly make a good frothy foam. I have read online from quite a few people who went to the Octopus Extreme (as opposed to something like a BM) and they certainly are pleased with what they have, and the fact they're quite a bit cheaper than the BM there's that to look at.
How big is your tank? I know they have the reduced footprint models (the *160? Or something) where they stick the pump under the skimmer, so that might work out if space is tight.
the tank is 220gal. i found this octopus used and a good deal. i dont have a skimmer and was thinkin it is gonna help. if the addition dosent cost much more.
Well a skimmer is definitely better than no skimmer, how useful it will be will depend upon your bioload and what model it is (there are literally a ton of different Octopus skimmers)