What I wanted to say was that I won't run an Octotank without a skimmer because of the pollution.
The tanksize which is published by the skimmermanufactor should be multiplicated by 2 (or 4). That is a rule for reeftanks with a heavy fish population.
For example: Use a skimmer which is produced for 800 l in an Oktotank with 200 - 250 l.
If you'll use a mudfilter or a DeepSandBed, the skimmer can be smaller.
An Oktopus is at the top of the food chain??? (Nahrungskette). They are very succesful preditors, thats the reason why it is not necessary for them to use the nutrients of thier food in a efficient way.
They are producing a very big waterpollution when they are eating.
To keep your water clean you need bakteria in your tank, therefore your tank needs place for the settlement of this bacteria (liverock and sand, here you can add an sumpfilter with mud). The bigger molecules need to be removed from the tank, that is made by the skimmer.
The manufacturer of the skimmer describes only the size of tanks for which you can use the skimmer. But they dont say anything about the number of fish (wich produce the pollution) in the tanks. If you have only few water in the tank and much pollution you have to clean it very good.
That is the reason why I suggest to use a 800 l Skimmer in an octotank of only 200 - 250 l.