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You can take your pick from these three form of filtration.. wet/dry, internal power filter like a fluval 4 or external canister filter.
The best way to use it it in conjunction with a sump and ideally i use a wet/dry with a large trickle tower. If you use a sump the equipment can all be put there and the octopus wont mess about with it. A sump also increases water volume.
A skimmer is a must!!!!
And fresh good quality carbon changes every couple of months or after an 'inking sesion' by the octo.
hope this helps... for more info have a read at the articles in the ceph care section of the site...